Yup, an accurate lighting system will be huge towards achieving realism. In addition to that you'll want your artists to be experienced with these types of physically based lighting systems so you can't create "impossible" materials or lighting setups.

Offline rendering methods such as path tracers already can achieve photorealism, but the techniques used there are way too expensive to apply in a real-time context.

Agreed -- Lighting.

Tim Sweeney of Epic Games and a graphs wiz every bit as good as Carmack--if not better--roughly lumps generational lines of rendering advancements as to how many bounces of light they simulate. In raycasters like Doom, light bounced just once off of a surface of the world directly to a pixel on your screen and nothing intervened in it -- light didn't come from anywhere in the world, it was just an ambient value, ever-present, constant, radiating in all directions evenly. In the first generation of polygon games like Quake or Unreal, light "bounced" twice -- light affecting a particular surface had a single origin in the world and each origin could have different properties, an ambient lighting factor continued to stand in for all the indirect bounces; pre-baked light-maps helped color-in the illusion of localized lighting and occlusion. For a long time, lighting advances came by increasing the number of lights in a scene, not increasing the number of bounces -- Even through Doom 3 was more lights, not more bounces. Modern games of today simulate ~3 bounces -- IIRC, the ambient light factor bounces off of one surface, picking up its properties, propagates that to a nearby surface, and finally the sum of this and the localized, two-bounce lighting to your eye. The coming generation--maybe today's bleeding edge--should make a good run at subsurface scattering.

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..

A parallel advancement has been Physically-based lighting, which gives materials a logical consistency like you see in the real world. In the past, materials were often bespoke and could have their knobs tuned to wildly different values to achieve an appearance consistent with the scene as a whole.

Realistic is a very misleading word the way we know it. When people say I want a game to look "realistic", what they really want is to have it look like it was filmed by a Holywood studio with special lighting setups (which may vary for different shots), makeup, particular hair styles that always highlight nice places and cover the not so pretty places, special lenses, and particular camera angles with a particular camera movement.

You know the phrase "even the girl from the fashion magazine doesn't look like the fashion model in the magazine" ? Same applies to "realistic" graphics, because people expect game's graphics to look like people in a magazine, TV shows, and movies. Which isn't realistic at all.

Therefore to look realistic we have to mimic what they do: Once we figure out the math stuff (proper BRDF, use HDR, Depth of field, wind effects, noise for a shaky camera effect), we need to setup lighting as in movie production (i.e. 3-point lighting is very popular), have the characters perform fashion-model-like walks for females, movie-like poses (3 point landing, anyone?), account for the 12 principles of animation, place the camera in strategic places, have camera shots change at the right times, have the depth of field focus what's important and leave what's unimportant out of focus.

Of course, high resolution textures, high polycount, motion capture and global illumination helps a lot; but it will only get you so far.

And of course, all of these "rules" can be broken. If you know what you're doing and know when to break them, it still looks good. When you don't, it looks crap. Just like crappy movies or your grandma's pictures (no offense to your grandma!).