I get that Gen Con is a big convention, yes. But why should it matter to the tabletop RPG player?
I went to Gen Con five times, and here's what I got out of it from a tabletop perspective.
The opportunity to play games that I didn't otherwise have. I like and collect a lot of games, but getting players to play anything other than D&D and the other majors is tough.
The opportunity to play games run by the games' creators. You can often get a lot more out of a game having seen "into the mind" of the designer, and them GMing a game for you is a great way to do that.
Access to purchase a bunch of games that don't come to your FLGS and are hard to find in general. A lot of the smaller games, you can find them if you know about them but that's a chicken and the egg problem; the dealer room at Gen Con helps you find them.
If you participate in Organized Play on a local basis, you can get a bunch of games in and advance your character.
If you have ambitions to get into RPGing in more than a pure consumer basis, you get to meet people, make contacts, etc.
It's fun, you get to spend 4 days gaming. A lot of people have trouble getting in more than a couple hours a week of gaming in the steady state. Heck, in smaller places people have trouble getting a group to game at all.
And in the end, it's a vacation, and travel... Vacations and travel are nice, and ones with excuses and people related to your hobby make it better than just heading to a random city where you don't know anyone for 4 days.
If you get 30,000 gamers all in one place, something cool is going to happen.
Big cons like GenCon offer a couple things that are special:
Awards There are at least three RPG awards ceremonies held at GenCon. The ENnies, of course, are a big deal. Lesser known are the Diana Jones Award (attended by industry insiders) and the Indie RPG Awards (usually an informal affair in the dealer room).
Amazing Art Small cons don't get the kind of art shows that the big ones attract. Don't get me wrong, there are some extremely talented artists at small cons, but you don't get the kind of enormous show that a GenCon can put together.
Games On Demand If you can find it, a bunch of indie gamer enthusiasts host eight or so tables somewhere, and you can go there and jump into an indie game with a generic ticket. There are dozens of games available, and GMs there who can run them.
Big D&D Events If you're a D&D fan, look for events like The D&D Open, where dozens of teams compete to be named the best D&D team. That's just one example. There was also the Convention Delve, where you can grab a 1st level character and play as often as you like, and Dark Sun Arenas. It's cool to share an experience like this with hundreds of other players.
Debuts Companies like to debut their latest and greatest products at the big cons. Want it first? Go to GenCon or Origins or the like.
Embassy Suites After dinner, the Embassy Suites Hotel (downtown Indy) turns into a gaming frenzy. The hotel has a pretty large cafeteria that they open up to gamers till around 1-2 AM. A lot of pick-up gaming happens here, often with people you just met. It's amazing. The Embassy Suites rooms are large, with a couch and table big enough for four or so people to game. A lot of gaming happens in quite, private spaces, too. Also, complimentary breakfast for those staying at the hotel: enough said!
The main advantage that I see Gen Con having over smaller, local cons is that it's one of the few big, national conventions out there specifically focused on roleplaying games. The gaming isn't necessarily better than a local con, but the national draw makes it a better environment for meeting people -- and perfect for meeting up with people you know in the gaming scene but who live scattered across the country. It's a good opportunity to reconnect with your old high school gaming crew that's long since moved out across the country, or to meet up with people you've met online through online games or forums.
In addition to other good answers here... The Auction.
It's the best-kept secret at GenCon, and it's been there every year since GenCon 2.
The best part, for me, is that I can buy almost any game or accessory ever made dirt cheap (except for newly-released stuff). And there's a fixed-price thing too, so you don't even have to sit and wait until something shows up on stage; just go to the Store.
Bring game stuff you don't use any more. The auction sells 'em for you. You get paid the next day. You can spend that money on new games in the Dealer Room (or use it to help pay for meals or whatever).
It's the biggest game auction in the world. They run 9a-midnight Thu Fri Sat. There's even a UPS stand right outside the place, in case you buy too much. ;>
Seriously. Check it out. Save a bundle, and turn old stuff into cash.
Big D&D events take place at GenCon year after year- Living Forgotten Realms always has a "special" adventure, and often a linked series called a 'mini-campaign'- last year's Mini took place in Returned Abeir in the Windrise ports. This years had 3 parts and took place in Raven's Bluff. One big draw of Organized Play is that characters are consistent: in other words, if you have a level 1 character at your first event on Thursday night, you can play that same character all the way through and end up with a character that is level 3 or 4 by the end.. and next year you can come back with your level 4 character and continue. LFR currently handles up to level 20 and there are plans to go into the epic levels. Where will this content debut first? GenCon.
Another big draw of organized play is that there's the chance to play alongside people from all over the country-- sometimes all over the world. The Worldwide D&D Championship (which is a scored, competitive team event with prizes) also takes place at GenCon, and then there are usually other events such as standalone adventures.
It's also important because of its provenance. I can't speak for how it compares in a contemporary context to other gaming conventions such as PAX, but I don't think that anybody is going to try and argue that any other convention is nearly as historically significant as GenCon. It's probably fair to say that RPGs wouldn't exist as we know them today without GenCon.
Mxyzplk is absolutely right. Let me throw in there the chance to play the games you do know in new ways with folks you've never met before, and that being around that much gamey goodness is an amazing spur for the imagination.
I know it's not technically "tabletop" but True Dungeon is extremely fun and very similar to TT RPG. Once you physically walk through a dungeon AS your character it can give you new ideas and a different perspective for your tabletop campaigns. As far as I know, GenCon is the only place they do True Dungeon...
A lot of gaming companies schedule their major releases/announcements to coincide with GenCon allowing you first access or pre-access to the latest and greatest in tabletop RPGs. On top of this many companies will also push to get at least some new product at GenCon so small numbers of new things may be available that won't be available in stores for months (even for people who have pre-ordered!)
Also to emphasize the point above about access to games' creators. Not only do you get the opportunity to play games run by their creators but many of those same people are willing to chat with you at their booths or at panels about their creative decisions, how they got where they are today, and just about anything under the sun.