Do remember which many do not that dx10 was all about a faster flowing api than dx9. Nothing really improved for gfx just the speed at which it was drawn. At least thats what i remember from a few articles when dx10 was coming out. Same with dx11 but should be better for gfx but again its about speed which should be good according to some articles. Even for dx10 cards which will benefit from the code optimises.
This is because most of the really good features of the best features of DX10 were ripped out before release, because Nvidia failed to get hardware working that supported the "good" features of DX10 they put pressure on MS to cut them out.
Assasins creed is around 10-20% faster in dx10.1, over dx10, because most of the missing features(not all) were enabled and ATi cards could use them, that 20% speed is rumoured to be a very VERY easy performance to gain from far far more effective multithreaded coding. Remember dx9 was out way before dual core cpu's, let alone quads, dx10 was supposed help games coded better to use multithreading far far more efficiently. Game dev's are saying 15-20% should be easy, some games could get as much as 50% more performance via multithreading.
People stopped using DX10 so much and stopped putting effort in as without those performance increases, the visual extra's didn't have enough spare juice to actually be utilised. AS in, people spent time and money and effort coding for the original dx10, but after it was gutted, the time and effort was largely a waste, some persisted, some didn't. DX11 will be an entirely different story, massive performance advantages, visual upgrades, improved efficiency.
THe only real thing DX10 had was soft particles(Bioshock is a great example) in dx9 you'll see smoke for instance clip with the floor and it looks like an edge which is unrealistic, in dx10 smoke can "blend" into the floor without the edge looking more like a gas and quite a lot better, small but lots of small things add up to look much better. IT also had less overhead for each particle so you could increase the number of particles, but with the missing performance improvements you could have more particles, but really had no extra performance spare to make that feature useable.
You can bet most games out there are dx10.1 compatible and can use lots of dx11 features, but were disabled due to dx10 being butchered, we've seen this in Assasins Creed where they were pressured to remove dx10.1 by Nvidia. You can bet most TWIMTBP titles are dx10.1 and have improvements that are disabled, you can also bet the second Nvidia has a dx11 card out that dx10.1/dx11 patches will be out for things like Crysis and without question, Assasins Creed 1(and 2 by then probably).