I can give you a few...
SSDs under WinXP gradually degrade in performance, because XP doesn't support SSD TRIM. On Win7, this is not an issue, so you don't have to wipe / reset the SSD / restore the operating system once a year.
Graphics performance of video drivers - I gained 20-30% performance switching from XP 32bit to Win7 64bit on the same machine, maybe even doubled performance. This was back when I multi-boxed EVE Online - I went from struggling to run 3 windows (at least one would only get 15-20 FPS), to being able to have 5-6 open (all with 40+ FPS).
The 32bit limit of 3-something GB of RAM is a bit limiting when Firefox is chewing up 500-800MB, Thunderbird is chewing up another few hundred MB, and a handful of other background tasks chewing up 40-50MB each. Moving to Win7 meant I could put in 8GB of RAM on the box, and make use of it.
Multi-tasking performance is just better in Win7 when compared to XP. Less hiccups / pauses / other strange slowdowns.
The window preview as you hover over the tasks in the task bar is addictive. Being able to see thumbnails of each application window makes it easier to pick which window to bring forward (another bonus for multi-taskers).
A bit more resilient then XP to being infected - not perfect, but a definite step forward.
We run Linux on the servers, but I'm quite happy running either OS X or Win7 on the desktops. Both get the job done well enough and stay out of the way.
(Running Win7 on a 2007-era Thinkpad T series, 8GB RAM, pair of SSDs, and only a dual-core Intel CPU.)