..dont forget far better 64-bit support, which is increasingly important with 4GB seeming to be the starting point for RAM these days.
I don't disagree that XP is the most widely used OS, nor do I suggest that 7 is a good choice for older hardware. We're still using XP at the office, our clients are still using it, and it's been around nearly 10 years. Win7 is making the rounds though, very difficult to buy a new PC with XP anymore, most home users seem happy to go with 7. I'm waiting for the day they ask me to install 7 in the test lab.
..and in all fairness, Amiga really only did take a couple of steps...