http://www.tesnexus.com/news/index.php?id=801 quotes an http://uk.pc.ign.com/articles/111/1112464p1.html with Todd Howard,
Presumably the id engine, being highly optimised to render stunning static worlds, isn't so hot at handling vast amounts of clutter. In fact, I doubt there's any game out there with the same level of clutter as Bethesda's RPGs (from Morrowind on) have. Not many, at any rate
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I actually wouldn't be surprised if id Tech 5 can handle clutter just fine (and definitely better than the other engines they've used). Keep in mind that it's made as a general-purpose engine, with its greatest strength being large and well-populated maps (Rage itself is an open-world game). And really, knowing who made it I'd be surprised if there's anything any other engine can do that it isn't at least decent at. Clutter's definitely not something I'd expect it to fail with though.
From what I hear, (I believe from one of the podcasts or interviews with Todd Howard or something) that they didn't lift anything out of id Tech 5, but some of the programmers behind the engine served as advisory roles while they created Skyrim's new engine.
If this is true, that could be pretty significant depending on which programmers we're talking about. id has some of the best in the industry (and, again, have the best in the industry).
But it's not always about the engine. The true beauty comes from the quality of models, textures, and animations you make. The engine just does those with as little framerate cost as possible
Also plays a role in how big and detailed those models, textures, and animations can be, the kinds of effects that can be used with them, and things like loading (if it's in there at all). Can also have a pretty huge impact on stability and development (the latter being important for modders). Engines don't do most of what most people here think they do, but they still impact a lot of things.