Fourth, Doom3 Ruled!! It may have been shorter than many would have liked, but it was fun, beautiful and didn't crash every 3 mins...
Actually D3 was mediocre as a GAME. It was visually stunning, very ground breaking, but as a game just didn't have legs. Pop and shoot, pop and shoot. Many people (including myself) got so bored they didn't finish it. It was beautiful, and it has alot of ground breaking aspects, but as a game it's only mediocre. As a tech demo especially of the ability to scale effects and visuals well even down to like 800x600 it's really great.
Fifth, NONE of ID's games have EVER used the DirectX API, and for the following reasons; DX has always been a buggy, difficult and expensive API. OpenGL is stable, feature rich, easier to program for and far more cost effective.
Except now Carmack praises the development tools for DX10 and Xbox using XNA. One of the things he praised most was ease of use.
Sixth, The DX10 "Crysis" trailers show very FEW improvements over the DX9 versions. The differences are only important IF you care. The vast majority of high-end gamers will think these effects are "neat", but not critical to gameplay.
Depends on how they are used. Critical to gameplay is also a tough thing to pick, some people will compromise alot of effects to get better speed for deathmatches, but for immersiveness, I don't doubt that DX10 will be the way to go, but whether that's exposed anytime soon is another story. Some of the new replication options could allow massively complex scenes with increadible geometry loads under DX9 that would be lower impact with DX10. It's like changing grass size in Oblivion; is it a game breaker, no of course not, but it does make it more realistic than playing in what appears like fields of wheat.
The thing to me is not whther DX10 will be good or not, it's whether it will matter near term. And like you mention only some people will care even then. To me DX10 is important but really won't be exposed until the second batch of games. At first just expect tack-on stuff at first.
Personally I'm more interested in the addition of things like Direct Physics alongside the added geometry loads. That should make the feel of the environment more realistic, as for the look, that will depend IMO more on the power of the VPUs, since shiny and pretty is already done.