...
There is one other game that I know of that is run in Gamebryo that uses the Isometric viewpoint... just a little known niche-market game called Civilization 4.
The biggest difference as already stated between UO and other MMOG is the sheer amount of data due to the sheer number of items being processed in-game. Most other, especially 3d-based games do not let you drop an item to the ground, also resource nodes are usually handled MUCH differently.
Another issue is as Dark Chronus mentions... the sheer difference in item artwork can be VERY confusing to relearn. Something like a chest, barrel, or basket is no big deal (although a LOT of the item artwork DID start out in what must have been a lower resolution setting that had to be refined), however, EQUIPMENT items such as qarmor and weapons had HUGE difference in them that made them VERY difficult (even for me, a KR enthusiast) to adapt to seeing and recognizing. Mauls in KR look like Warhammers in 2d, Viking Swords and one of the versions of Long Swords are VERY close and easily confused, I still believe the graphics for Studded Leggings and Ringmail Leggings were swapped based on look and color of the two sets of armor comparatively, the list continues. Only through rigorous BOD filling has it really started to "make sense" to me in terms of item artwork to the item it represents in KR vs what it did in 2d.
For the updating of KR to SA as a client, there are some good things that have been announced though... characters and creatures will highligh again, hotbars and health bars will be dockable, the UI will be fully movable (and I know someone who has worked out the ability to anchor them as an option, so you can set up your UI, anchor each piece and not worry about dragging them around unexpectedly), the memory and animation footprint will be smaller (which means it will run even better, and for some of us, it runs pretty freaking fast), and probably plenty more to come (considering they've had about a year and a LONG list of feedback to work with).
So we'll see what happens.
I'm still skeptical about an actual 3d client for UO... the history of 3d and Ultima has not exactly been the best in the past decade.