The last couple of years I have used a messenger bag and will most likely stick to it even with the negatives of it (all the weight on one shoulder and sore back from it). But I don't want to do a traditional backpack because I don't like the idea of my bag being behind me and not being able to see if someone tries to get into it. Yea, I am paranoid. Blame it on me being raised by a cop LMAO. So that whole thing is still up in the air as of right now.

As for what I carry in whatever bag I end up bringing...
- Medications
- Hand Sanitizer
- Travel sized deodorant
- Travel sized toothbrush/paste
- Hair brush (have one that folds up)
- Pens/pencils
- Binder with all my parking/event information, dining information (I have a food sensitive medical issue so I plan ahead), notebook paper (I am a writer so I like to keep stuff like that on hand)... it's more light weight then it sounds, I swear.
- Snacks
- Filtered water bottle
- Dice (most likely the heaviest thing I carry haha)
- Charger
 

It's not "IN" my bag, but I do recommend boosting your immunity starting two weeks prior to Gen Con. Some people recommend Airborne, but you get the same active ingredients from One-A-Day Immunity Boost multi-vitamin gummies, and for less than a quarter of the cost of Airborne. I take two per day starting two weeks before the con. 

If you have a portable battery charger, I'd recommend that instead of a wall charger as wall sockets may be hard to come by.

Other items: 
Dwarf repellent. Elf song sanitizer. Vogon poetry translator, Boardgame paper cut repair kit, Beholder eyelashes.

There are two modes:

No bag:
-Pencil & Pen
-Mints
-Small notepad
-Pocket charger + condensed cord
-3-4 Dice pending game system
-Water bottle in hand

Game bag/satchel:
-More dice
-Large battery pack
-Snacks (fruits/granola bars)
-Whatever game book(s)/Miniatures (pending events)
-Space for dealers hall goodies

...oh, and orange juice works too.

But advice is perfectly fair, since anyone can pay twice as much outside of the official Gen Con Advice Portal to get all the advice they want!  (EDIT: That makes a lot less sense since several replies were added while I was making my post.  Oh well, I'll survive.)

For a serious answer, though, I'm still trying to narrow it down.  I'm not currently happy with any of the bags I own - I used to use a Thinkgeek Bag of Holding, but I don't need nearly as much space as I thought I did when I bought it, and the ubiquity of it means I keep seeing others' bags and thinking they're mine (or worse, vice versa).  Thinking of selling that one and buying a sling backpack, although I've never really had a problem using a messenger bag.

As for what's in it?  Here's my current plan for this year:

-Fresh socks
-Empty, folded shopping bag
-Wireless earbuds
-32oz Nalgene bottle
-Aleve
-Deodorant
-Hand sanitizer
-Contact cards
-Granola bars & beef jerky/jerky bites
-True Dungeon tokens & party card, if I'm playing TD that day

I'll probably end up carrying the dice tin after I buy one.    My pockets will have my phone, notebook, wallet, pen, and Chapstick.

A few things on my 'maybe' list that I'll probably just skip:

-Comb (depends on how long my hair as come August)
-Battery charger (used exactly zero times last year, but my phone was brand-new at the time)
-Kindle (if I have a book I'm reading on it that week)
-Deck of cards (you can always find SOME use for an old Bicycle deck)
-Magic deckbox & sleeves (in case of impromptu booster draft, although I do find the box useful if I buy singles in the vendor hall)
-Event program (I feel somehow naked without it, but rarely flip through it during the day)

That should cover all my bases.  Looking at the list again, a whole backpack might be overkill, but I don't know of a good way to carry anything smaller.

backpack: dice, pens, pencils, small paper pad, whole fruit (oranges more than likely), reading glasses (the font seem to get smaller each year), full sized program, two sets of earbuds, usb cables, battery charger, water, extra strength headache pills, sun glasses (for those random forays outside), game tickets, granola bars, card game or two.

The heaviest things are the dice, fruit and water. The backpack still has room for game/shirt/book purchases. 

As little as possible. 

• Dice
  
• Pens & pencils
  
• Notebook
  
• Drugs (Advil and such)
  
• iPad - because it has a lot of PDF rules on it
  
• Water bottle - it is important to stay hydrated
  
• Business cards  
  
• Cloth carry bag - something to put purchased items in that makes them easier to carry
  

I agree with the basics that have already been mentioned.  Nothing that I don't know I'll use makes in in the bag.  I use my Tom Bihn Synapse 19.  It's my every day bag anyway so I just switch out stuff.  In addition I like to bring one of those small folding hand fans.  Temperatures can fluctuate wildly and I always want it on Wednesday night in the ICC.  Also, I prefer wet wipes over hand sanitizer that way in case I need to clean off a grungy table I'm gtg.  Hive pocket is a must for me too.

I try to travel as light as possible, and I got lucky enough to be staying downtown, so I'll ideally have

• dice for the next game in my pocket
  
• phone in a pocket
  
• notebook in a different pocket
  
• pencil/pen in the badge holder
  
• event tickets also in the badge holder
  
• one of those drawstring backpack things which fold up very small in yet another pocket
  

As of 2015 I've started stripping the plastic insert out of the Gen Con commemorative dice sets, so that I can fit other things in there as well - If anyone knows of a small but still usable pen or pencil that could fit in one of those tins?

Yard Gnome and a rubber chicken, for obvious reasons.

I've had the thought that I could stick a robbers mask, a revolver (an obviously fake one, to not violate the rules!!) and an empty white sack with a "$" on the side, then just empty out my bag every time I sat down to play... I'm just not certain anyone blink an eye at it ("Oh... game props... well, whatever...").

Messenger style bag (aka Manpurse, aka Adventurer's Satchel) with notebook, mechanical pencils, dice, phone charger, reading glasses (cuz I got old), a long sleeve shirt or sweatshirt (because once-in-a-while you have a game in a room in which the AC is on overdrive) and snacks.

(I'm also going to pack some extra advice this year since it sounds like there might be a shortage. I'll try to scalp it on the sidewalk outside or to people waiting in the long registration line.)

I am thinking of getting the UPGRADED Bag of Holding from ThinkGeek (if I can find it at my local store) http://www.thinkgeek.com/product/aaa5/
 

Well, I have to get a new backpack for this year, but that will be holding the following on any given day:

-Any rulebooks I need for a game I'm running/playing in that day
-The binder holding the adventure I've written (if I'm running a game that day)
-The binder containing my character sheets and wargaming army lists
-Dice bag
-Mechanical pencils

I'll also have a miniatures bag containing my miniatures if I need them that day.

That advice-shortage exchange was fantastic and definitely made me laugh out loud.  Kudos all around.

The only suggestions I have for what to carry with you not specifically covered already: to go with your water bottles, grab a variety pack of powdered drinks; there are some brand names now like MIO or whatever but the variety of water-mixers you can buy, including some that are specifically portioned to go into a full normal-sized water bottle, is pretty stellar.  Not that there is anything wrong with just drinking plain water :)

Relatedly, I have occasionally lamented not having sweetener or condiments on me those rare times I get something to drink or eat in the convention center, and a dozen packets of sugar substitute, hot sauce, mayonnaise, and mustard or whatever do not add much weight :)

Finally, even though I freely admit I have still never used them, I started bringing black AND silver Sharpies a few conventions ago, since getting autographs on a dark surfaces will obviously not be possible with normal writing implements.  But uh...yeah I think I first brought those the first year Gen-Con seemingly gave up on having broader pop culture guests.  And I can only have so many signed Elmore prints!

Several things I would recommend for people to bring are:

1) Powdered drinks if you need them for water. An item that I just found is Sodastream has flavorings for water that do not have the aweful after taste of items with Sucralose or Aspartame in them.
2) A large battery for charging devices. Our Walmart near us just had a 12000mAh battery on clearance for $12. That will be coming to Gencon with me.
3) Three way plug so that you can plug multiple devices in at the hotel or at a plug near you. Hotel rooms do not have alot of outlets and it may be hard to charge all your devices without the plug.
4) I travel for a living and one thing I have found recently that was an awesome find here on the boards was a binder clip or two to hold the hotel curtains together as they never want to stay by themselves.

The Bag I bring:  http://www.duluthtrading.com/store/product/the-beefcase-large-canvas-briefcase-44402.aspx?p_redirect=1&p_keyword=beefcase&p_origin=mens-home

What I like about the bag is that it can easily convert to backpack depending how my shoulders feel.  Otherwise, I normally travel light.

Outsides of dice, a small bottle of aspirin and writing utensils, my iPad (stocked with PDFs), the most important thing I have with me is my Trip Book.   I print out my flight reservations, hotel reservations, any dining reservations, GenCon gaming schedule, maps (including dealer hall) characters that I play in such games as Shadowrun Missions & Adventurers League, and blank scratch paper...then get it all bound as a notebook at  Staples.  Very handy.

I'm not so much bag dependent as I have been in the past but  not going to rule it out this year. Depending on cosplay choices might limit my bag options since I have been indulging in costumes more. My first year my bag was the free bag given to all attendees. As the con progressed I collected free items in the dealer hall. Mostly flyers and such. At some .point I think I had a water bottle.

Very small messenger bag for me, my friends have dubbed it the "bag of holding" due to my ability to stuff large amounts of odd things in it.

What I stuff it with...
Chopsticks- forks are sometimes hard to come by at the trucks
Bandana
Notebook-full of maps, tickets, vendor info, etc.
pencil
water bottle
granola bars
camera
meds
magnifying glass (friend always forgets her glasses)
safety pins (you never know when you might need one)
chapstick
extra bag for purchases