Adaptive animation is another of the current frontiers that builds on realism. After that, probably AI/Behavior that results in more than a simple choice between pre-canned responses that are blended at best -- something more natural than that will be needed to cross the uncanny valley once we reach realistic-appearing humans in real-time..

+1 on mentioning animation. It's too often overlooked. Maybe we shouldn't talk about "photorealism" but "movie-realism".

On that note, lighting is not only about bouncing in the world, but also how light interacts with a camera.

If it weren't for the ubicuously multiplayer choices, I'll say that artificial intelligence has a too long road ahead to be overlooked. Definitely, AI needs a boost.

This is a rather general question: what defines the realism in a game? All I can think about are texture quality and higher polygon count. I'm no professional designer, so I'm not sure what goes into a scene that makes it look real(other than the stated examples). I've just recently started 3D programming, thus I won't be making the next Battlefield anytime soon, this question is solely asked for my curiosity.
Note: I'm not asking for code or language-specific examples, just the general variables that differentiate a game that looks unrealistic and a game that looks realistic, such as a lighting system.

Here's a post-mortem from a PS3/Xb360 game I worked on:
https://dl.dropboxusercontent.com/u/13063685/RLL2f.pdf
The slides at the start show the before/after graphics quality between the original game and the sequel (this project). Most of the other slides are all of the work that went into achieving that quality increase. As a graphics programmer, that's basically my diary for 6 months :lol:

But there's some good points above -- pretty much everything in this PDF are techniques that make each individual frame look better, but not necessarily making the game *in motion* look better. This game got mixed reviews for graphics (some people thought it looked great, other people thought it looked absolutely terrible)... part of those complaints were because the animation/AI was not up to scratch.

For the actual state of the art though, skim through some reputable presentations :D
http://advances.realtimerendering.com/s2016/
http://advances.realtimerendering.com/s2015/
http://advances.realtimerendering.com/s2014/
http://advances.realtimerendering.com/s2013/
http://advances.realtimerendering.com/s2012/
http://advances.realtimerendering.com/s2011/
http://advances.realtimerendering.com/s2010/
http://advances.realtimerendering.com/s2009/

I recall from years ago one of the authors said "Never let them see you pop" which is in reference to blatant LOD shifting in a game/simulation