I'll echo similar sentiments - if you only ever go to a single convention just to see what it's all about - go to GenCon.
While local and smaller conventions can sometimes focus on a small sub-set of games, or even be rather particular in what they host (due to local resources), GenCon is the "big one" and as such covers ground you may never have thought of.
For instance, did you know that often at GenCon there is a game of "Evil Stevie's Pirate Game"? 'Tis a pirate game based on using Lego pirate ships and running around fighting your fellow pirates for fame and gold.
Too, there is a huge boardgame hall, a miniatures hall, and of course, RPGs out the wazoo (many of which are for people to learn the game and require no prior knowledge), of every shape and type (and version). And the dealer's hall has more stuff to buy than you can shake a stick at - not to mention companies are often running demos of their new games, right there at their booths.
Aside from gaming, there are panels and seminars too! You can find panels covering what's new from a company, as well as sometimes covering a general topic that is opened up to the public (like a public forum for, say, D&D). The seminars can cover everything from improv DMing to creating memorable challenges in game, to how to write for gaming companies - really the topics can cover the gamut.
And GenCon has special events as well. There is the charity auction, the normal auction, a special breakfast, costume contest, Paint N Take (miniature painting), and the ENies.
And all that are just the official bits. There are tons of pick-up games. It's not uncommon to see gamers getting their game on in hotel lobbies, breezeways, and anywhere else.
As I mentioned, many local cons can be constrained by their resources. However, of the ones I've seen they often have similar offerings as GenCon, though in much lesser degrees - and sometimes boardgames may heavily outweigh RPGs - or special events may not be present at all.
I guess all of that was simply to say that yes, a convention is what you make of it. Try new things and keep an open mind to new games that may interest you.