Hey there,
For a while I've not had the time to do any mapping or play any games (let alone Inf), but the board crash rewoke a thread I was subscribed to so I thought I'd have a look round. Since I used to design Inf levels I thought I'd add my $0.02 to this discussion.
What UT3 seems to lack (so far as I am aware) is a decent level of detail (LOD) system to allow big open maps. Look at the levels and you just don't see big wide open outdoor spaces (without lots of fog). By contrast, something like Far Cry worked a treat (no doubt Crysis too - haven't had a chance to install this yet), because e.g. bushes that were far away are reduced to much simpler shapes.
I think you need a decent LOD system to do said large outdoor maps but also big urban levels with freely enterable buildings that have complex/furnished interiors. The problem with this sort of urban level is that the optimisation is screwed - if the buildings are enterable (and the interiors can be viewed from outside) you can see a massive amount of objects at any one time, and nothing is effectively occluded.
Another mundane problem is that few engines allow buildings to be constructed with realistic interiors, because you always have to 'scale up' to avoid the AI (and human) players getting snagged on objects and walls.
I think large outdoor levels and large complex urban levels are the backbone of a decent military simulator such as Inf 3 aspires to be. If you have very linear, constrained levels you end up with a venue for team deathmatch and spawn camping and little else.
I think Battlefield 2, for example, did very well on the outdoor levels, but not so well on the interior/urban front. Some of the levels are nevertheless very evocative, which I respect greatly as an (ex) mapper. COD4 also has some lovely mapping work and very creative level design (if controversial in places).
As an aside, as engines get better, the 'rough edges' of my work show up more and more - I had a level I was doing for UT2004 but getting the models nice enough was just too much hard work. The current generation of engines are probably beyond my means, unless I can find people to fill in the gaps in my abilities.
Anyway, my point after that long, rambling post is that if you're looking for a new engine, you may want to ensure that your chosen engine copes (or can be made to cope) with large, partially occluded and 'busy' levels (e.g. by using decent a LOD system).
If anything can get me back to proper mapping it will be the promise of being able to realise the kind of urban maps I wished I could have done for Inf. There are others (who may be better placed to do so) who might feel the same way...