The complaining is more visible because there's forums now, we're all required to be at the computer to try to get events (so the forums are right there to receive those complaints), and because attendance is higher. (I doubt these suggested reasons are exhaustive, but they're three large parts of the equation.)
There was plenty of complaining before we had an online registration system, but it was talking with real people who were in the same boat while we all waited in the Will Call line (where you perhaps got nothing other than your badge) and the Tickets line (where you searched the physical book and filled out plenty of slips of paper with the numbers and names of events in the hopes that some of those tickets were still available). I called it Line the LARP: The First Event Everyone Plays At Gen Con. I played it every year in Milwaukee.
Over the years the online event registration system has had ups and downs on opening day. Some of the downs were pretty big, like the year where it crashed so badly they had to reset everything and redo event registration the next day, or the year where a discussion of the ongoing yearly failures of the registration system (and a big misunderstanding about how people who weren't able to connect to the servers at all wasn't part of their data) was the catalyst for the VIG program. Things have improved since then, thankfully.
Personally, the only issue I have with it is where the system does not work as intended and returns errors or just stops responding. If everyone who was trying to get tickets could click the button, have the server return a proper response, and end up in the queue, then I would be content. That doesn't happen currently; in the last three years I've had it work fine once, not get a response once, and return an error once. It doesn't seem to be localized either, based on others posting here and elsewhere. It's hard for us to know how many people are actually affected by these issues as our posts aren't solid data by any means, but the goal should be to reduce these sorts of seemingly solvable technical issues caused by the things under Gen Con LLC's control to none.
Note also that this issue is not and has never been directly about what events you get; it's about system failures and other technical problems not being the reason you had no chance at being early in the queue. If 10,000 folks want to have a first wish list processed at the time registration opens, then they should all have a 1 in 10,000 chance of being first. And, of course, a 1 in 10,000 chance of being last. Been there!
Even if the current system worked perfectly, there's still improvements I would like to see to how it works. Some of the things we've suggested (with discussion happening in other threads) would likely alleviate the pressure of "you must be at your computer at this time so you and everyone else can try to connect to the server within the same couple of seconds" that contributes to the errors and no-responses and thus might solve the above issue on their own. Other suggestions are about how the chances at events are parceled out. Not surprisingly, not everyone agrees on if or what changes should be made, and that's fair. In the end, it's Gen Con LLC's decision on how to spend their resources.
The big downer is that I'd probably put more time into discussing improvements if we hadn't been mostly posting the same things for the last 10+ years. What was that definition of insanity again? :)