Just read a small remark in Greg's post about Europa Universalis III that I found interesting:
3D? Oh yeah, everything has to be 3D these days. But from what I can tell from the screenshots, this means that the soldiers standing atop your provinces are 3D models instead of sprites. That is to say, no impact on gameplay, and really, why bother.
I think 3D graphics can have many advantages that do not impact gameplay, either positively or negatively - advantages that make it worth doing in my opinion.
Personally, if I were to make a game, it would probably have 3D graphics with a 2D camera. Why? Process intensity. The computer knows something about what it's displaying, so it can manipulate it. With a 3D model of a soldier, I can re-use animations, I can change the colours of clothes, swap out pieces of clothing and accessories (weapons, typically), I can construct random soldiers out of individual bits. (Spore is a good example of what you can do if you take this to extremes.)
This is hard to do in 2D. I've seen the development of pre-rendered / hand-pixeled games. The Settlers 2 (not the remake... damn I'm old) had insane amounts of tiny tiny bitmaps. Several directions per settler, several animation frames for each animation, and each settler could carry any of, what, a dozen? different objects. Custom graphics compression and display routines had to be written. One of the later Settlers games basically exploded the repository that was used to hold the data. A similar thing goes for role-playing games, where you often want to re-use enemies, give them different colours and weapons, etc. In some of the RPGs I worked on, we changed colours by remapping the colour palettes. It worked, but it was limited. And of course we had all of our 2D characters in 4 directions... all hand-pixeled.
None of the advantages of 3D I listed above have a direct impact on gameplay, but they can give the game designer more options because production limitations are reduced. And being able to dynamically change graphics can make interfaces clearer.