Vista x64, just off the top of my head:
Better context switching with WDM offloading visual rendering to the graphics card.
Built in search is not just for documents, type event for the event log services for services manager etc. useful for finding xyz utility you use rarely.
More refined control of service start-up ie Delay Start, delays a service for a few minutes as not to bog down boot times. great for sql server and others.
UAC allows you to give admin permissions for the tools you need, while allowing you to test your code with lower permissions.
Shift+Right click on a file select "Copy as Path" very very awesome.
ReadyBoost, stick a 4 gig usb drive in and forget about it. Will be used for cache for superfetch and random access io paging, low memory systems this is pretty noticeable ie laptops.
IPv6 - you'll want it in a year or three
All around more secure.
My personal experience:
My work machine is a quad core 2.6 ghz running xp x64 and feels slugish and slow to my Vista home machine, a dual core 2.0 ghz. Visual studio is up in under a second, project loads in a few seconds, compared to my work box where it can take up to 30 secs.
"I am just uneasy about using Vista for my main dev system given its history"
It's history is it's been better than xp since it was released. People like to talk smack.