Small changes that would significantly improve graphics

Although we're close, we're still not quite there yet with photorealism in games. There are certainly technical hurdles, but there are also aspects of game graphics that can be slightly modified which would make the game look significantly better without raising the hardware requirements.  
 
For me, the most significant such change would be the texturing of wood (and wood planks to be specific). Although developers have mastered water and are making headway with foliage and weather, the texturing of wood in plank form hasn't changed for nearly a decade. When you look at a piece of wood in real life, it's bumpy, coarse, and has several imperfections. When you look at wood in a video game, it's perfectly smooth with very little imperfections, or characteristics at all for that matter.
 
 

More colours! I'm getting so tired of all the brown and grey colours in games. So probably more colourful design, and more riskier concepts.
But these are changes, that can be quite dangerous to do, in terms of sale and getting the mainstream market hooked. Mirror's Edge didn't sell that well, Brutal Legend hasn't sold that well either, but it's nice to see a game like Uncharted 2 ^^
 
This isn't probably what you meant, but I believe that this could improve the overall looks of games, if you what I mean :P But facial expressions is something, which is done pretty poorly in some games, looking at you UE3, and that's something, which needs an improvement

" For you, what is the most important change that should be implemented in current graphical design that would make the greatest impact on realism without significantly affecting hardware requirements?  "

Crysis is picture form still looks daaaamn good (for something that came out in 2007, it holds up in the visual department), but it always bothers me to see a nice AA'd image of you fighting guys and the foliage, trees and water look almost photorealistic and then........you see a rock, and it's really badly textured...and it immediately takes you out of the moment. 
 
the point here is, i wasn't bothered by the fact that the rock was low res, i was bothered that it didn't match up to everything else texture wise, it was like going from HD to sub-SD, you see 5 really clear things and 1 really blurry thing and it doesn't meld in with everything else, it's like playing a game of pick the weird one out.
 
To answer the question, make sure that most textures (atleast in a specific enviroment) match up in quality, and i think games could use more colouration (you may think the world is brown and grey...but grass and water ain't the only thing with a bright colourscheme out there).

" ...texture quality doesn't take a big toll on the machine as most benchmarks will tell you... "

I think more immersion of colours and textures, they just don't blend well together. There's always a clear line where the grass stops and the road begins, the rock's edge and the ground. That needs to change.
 
Also, particles, you can never have to many particles. We're seeing some good bullet impacts on ground these days, they need to keep that up, maybe characters running creates dust?
 
Having said that, there are exceptions, for example I think LittleBigPlanet looked amazing, but it wasn't technically a graphically intensive game.

" @Geno said:

" ...texture quality doesn't take a big toll on the machine as most benchmarks will tell you... "

I'd always had the impression that dealing with memory allocated to textures was one of the biggest challenges in game development (not that I know anything about it), hence the common practice of re-using a lot of lower quality textures, especially in larger scenes - I'd imagine wood, being something you don't really focus on, typically doesn't warrant using texture memory that could be used on reducing load times, etc.  I suppose techniques like the use of decals are often used to reduce the repetitiveness of textures (Source games), but memory still takes a hit - I agree that it's usually really obvious when a developer has to skimp on environmental textures - but I don't think it's something that's 'easily fixed' or a 'small change'. The megatexture looks like it might start improving things, but it would seem it's generally a hardware limitation - throw some more memory and faster drives (SSDs) and we'll start seeing improvements in techniques like texture streaming. "

Shadows.
 
I hate when today's HD games have shadows that look completely fake. They are either too dark, or seem to be made out of square pixels, and are jittery. It's the same problem that made the Playstation's polygons so unpleaseant.
 
Also, framerate.

To put it simply: it depends.

Crysis is picture form still looks daaaamn good (for something that came out in 2007, it holds up in the visual department)

I for one, would like less graying out or blood on the screen or whatever, when I am getting shot or shoot someone. I find it really annoying, distracting, and kinda unnecessary. Also motion blur needs to be less noticeable. I know in really life if I'm running/sprinting, there is a bit of blur, but some games just put way too much emphasis on it to the point where it is distracting. I don't know how much these would impact an improvement in graphics, but I kinda have pet peeves for gross overuse of that.