Minecraft was also a great achievement in terms of game design and gameplay, yet had simple graphics, and consequently was easily moddable, which greatly contributed to its popularity and success. But is was considered "AAA" when it was released, and is it considered "AAA" now that Microsoft bought it (even though it's essentially the same game)?
So how important do you think fancy graphics are to the definition of an "AAA" title, versus accessibility (in terms of how many people can play it because of its lower hardware requirements, and how many people can mod it because of its graphical simplicity)? And how important are game design, gameplay, popularity, making money and other issues like moddability to the definition of an "AAA" title?
Another way for a game to achieve easy (but more limited) moddability without limiting its graphical complexity is to support advanced built-in tools for user created content (like Spore for example, which has advanced built-in specialized tools as opposed to supporting simple generic third party tools).
Subsequent versions of The Sims had much fancier graphics, and much more advanced built-in content creation tools (like create-a-sim), but that made it harder to create content outside of the game (because objects were 3D meshes instead of 2.5D sprites, and texture maps for character meshes were not nearly as simple).
The original Sims 1 team (which I worked on) only had four core programmers, but it was developed over many years before the point that EA bought Maxis and put more people on shipping it. So The Sims 2, 3 and 4 were much more typically AAA-ish, and had vastly larger teams working on them. (The Sims Studio became one of four major sub-divisions of EA.)
I think small indie teams would be better off focusing on game design and gameplay instead of fancy graphics.
What I like about these games is that they're immersive, without being overly long or tedious. You can play through Portal in a few evenings, without feeling like you're missing out on hundreds of side quests. The gameplay is tight, fun and innovative.
However, if you have a friend who likes first person shooters, you could play through one or more of the Gears of War games in cooperative split-screen mode. I hate, HATE, HATE playing FPSs on consoles, but I had a blast playing Gears in coop. It was the first console FPS I'd ever played where
* the gunplay felt good
and
* I didn't find myself longing for a keyboard and mouse after two minutes of play.
Indeed, the combat works really, really well with a controller.
I don't know how the game has held up over the years, but it was lots of fun back in the day.
If you like, change FPS to "shooter" everywhere it appeared in that comment.
AAA game (typical example: Skyrim):
* Sells for $50-$60
* Has cutting-edge graphics and (generally) runs on the newest generation of hardware
* Is available on most/all gaming platforms
* (Nowadays) Usually has some celebrity involvement (Skyrim had Max von Sydow, Christopher Plummer, Michael Hogan. Oblivion had Patrick Stewart.)
* 20+ hours of gameplay
AA game (typical example: Far Cry: Blood Dragon, Call of Juarez: Gunslinger):
* Sells for about $25-$40
* Made with last year's AAA engine
* Runs on last year's hardware
* Typically available on fewer or only one platform
* 8 - 20 hours of gameplay
Anything below that is either a classic game remastered (for example, the recent Homeworld remaster), or an indie title.
Frequently a AAA game (like Diablo III) will have a AA game come out at the same time to compete with it (Torchlight II.) In a lot of cases, like Torchlight II and Call of Juarez: Gunslinger, the AA game is actually a far superior game to the AAA games its competing with. ;)
Sometimes the sequel to a AAA game is a AA game, I'd say that's the case with Wolfenstein: The Old Blood.
Some games, like Portal and Portal 2, straddle the line. But I'd slot the Portal games as AA.