One aspect of games (RPGs, specifically) that can greatly affect the "suspension of disbelief" for me is the ecology of the world. Everything should have its place in the greater whole, and there should be reasons behind the way things are.
Asheron's Call 2 is an example of how this was not addressed (among other aspects of the game, but I won't go there in this post). One of the minor races were the Mimbu, and they were all hostile. They obviously had some level of civilization (if only at a "tribal" phase) as they had equipment. But there was no explanation on why they were hostile. I mean, seeing how many other hostile races are in the area, I would think the last thing they'd want to do is make more enemies. To my knowledge, that was never explained.
Another example are the giant beetles outside Freeport in EverQuest II. What do they eat? And what eats them? One would expect a creature of that size would make a significant impact on the local ecology, whether eating plants or animals (though I suppose the stone beetle could theoretically be eating rocks, so I guess there's an explanation for them).
Also you'll rarely find sufficient farmland in any RPG. Granted, it would probably be boring to run through crop fields of the appropriate size for the cities they're supposed to support, but it's ridiculous when two small farms are somehow supposed to generate enough grain for an entire city (not only for bread, but also for beer)!
On the other hand, Guild Wars 2 developers have addressed these kinds of issues with some of the minor races of Tyria. The Quaggan, the Krait, the Jotun, the Skritt, and the Grawl have all had significant thought and planning involved to make them unique. Not only does this help to elevate them from "generic hostile creature" to something more distinct, it also sets the tone for interaction with them and adds depth to the world.
Dragon Age: Origins is a good example of certain aspects of an ecology being made a non-issue by design. The game's explorable areas are limited to certain points of interest, and the map is used to travel between those points. While this generally makes the game world feel smaller, it allows the game designers to focus players' attention and cut out less exciting (but important) areas without omitting them completely (such as the farmlands necessary to support a city and surrounding region).
A lot of these details don't matter as much in a single-player RPG; they'll help add depth to the world, but you're generally working within a "snapshot" of the world. However, when you move to a persistent world such as an MMO, where there needs to be a sense of continuity and depth to create a believable and consistent world which will keep players coming back.