What makes an "AAA" game (and its engine/technology) so special? Why are they so hard to make, take so long to make, etc? Why can't an average basement dweller write an game/engine with graphics that are just as good/realistic? Take Crysis or BF3 for example. If someone knew the techniques, algorithms, and everything else needed, what would stop them?
1) Creating Assets What "looks good" has a lot to do with a combination of technical features + artwork. Once you implement a i.e. a displacement map shader, you need to find a good artist and provide him with the tools to generate said displacement map. For an AAA game, the team involved in creating the assets is huge.
2) Performance and resource budgeting It's much easier to make a feature that runs a demo scene at 60fps on your beefy development box than to run the maximal scene in the game at 60fps on your minimum specs (and consoles)
3) Time and Effort I'm not a professional, and I have no numbers for AAA games, but here's a useful number: http://www.ohloh.net/p/doom3/estimated_cost Doom 3. 174 person-years.
4) Nothing, really? If you're just looking to implement cool graphics features, go ahead and whip a tech demo together! I saw this video a while back: http://www.youtube.com/watch?v=9N-kgCqy2xs or just youtube for random Unity3D or XNA demos or follow indie studios like Wolfireand you'll see that people outside the AAA game world are putting together great things.