I love Vista, it is so much better than XP that it's not funny. Its much more stable, things make a lot more sense, a lot less just weird bugs. I've only had a few issues with compatibility and one of those wasn't really a compatibility issue.
Some notes:
Vista does not use up more memory, at least not as much as you think. Instead it uses a technique called SuperFetch that caches takes into memory and keeps them there till you need more memory for use. That's why it might say 1GB in use but in reality as soon as you fire up a memory hungry task that'll cut in half easily and you won't notice. This is the reason why Vista feels very snappy and responsive. It is a great feature, to bad people don't know about it and therefore think Vista is taking up double the memory when its not exactly doing that.
You can disable UAC by bring up msconfig (run-> type msconfig) and going to the Tools tab and then scrolling down, select Disable UAC and then click Launch and reboot. If you find UAC to not be the annoying then maybe you shouldn't worry, but I find it amazingly annoying and therefore it was disabled right away for me. Your computer is "technically" more insecure this way but I can live with the "fear". You can either search for the run ability or you can add it to the Start menu by right clicking the orb and choose properties and then Start Menu tab and select customize, then enable Run.
Keep the Start menu organized. I tend to dislike the new design and how everything is on top of each other. The first suggestion I'd have is create a new folder in there called Window Apps or similar and then put all of the included programs into this.
If its a gaming system or anything dealing with performance use, look for updated drivers monthly. As of now there is still a very slight gap in gaming performance between XP and Vista, this gap is closing tighter and tighter with each new driver release. Vista has a new driver model and some interesting way of doing things underneath, the performance will eventually come up to par with XP, maybe even surpass it at some point.