What features make the game seem more realistic as a fantasy environment? For me it's:
2. Character is affected by weather (e.g., can see breath in cold air);
3. Sounds match the environment (walking in muddy earth sounds different than walking in leaves);
4. Footprints in sand, snow, mud, etc.;
5. Character is animated while idle (eyes look around, chest moves with breathing);
6. NPCs keep busy, and are not merely mannequins;
7. Breezes move tree limbs and foliage about;
8. Lots of little critters (birds, squirrels, moths, and such);
9. Towns have ambient sound appropriate to a populated area;
10. Detail in houses and stores, with curiosities to look at - not just an empty dwelling with a counter.
I'm sure people can think of more. And I'm equally sure some people feel such things are not important to them.
EQ1, EQ2, SWG, SWTOR, GW, GW2 CoH, CoV, FFXI, WoW, CO, War,TSW and a slew of free trials and beta tests