Vista has lots of improvements in it. Mostly for the corporates admittedly, but is a step forward from XP.
Yes, it needs lots of resources, but then so did XP and W2K before when they came out. XP will run faster on single core machines than Vista will on same hardware, but Windows 3 will run even faster - not that I notice anyone wants to run that

On modest hardware, Aero needs to be turned off. Not only does it need decent video, and reasonable CPU, but on low spec video cards and integrated ones, you get the double whammy of the video card needing to steal RAM.
Vista is more secure than XP. Its built on the NT kernel, and as from 2002, MS have been proven to use better coding practices - W2K3 server is proof of that. All OS's have flaws, but MS have improved dramatically lately - the CERT announcements prove that. Linux normally tops the list of new vulnerabilities now, though IBM's AIX and HP-UX have taken a battering over the last month.
Solaris, arguably the best of the mid range Unix, tends to get more vulnerability notices than Windows as well - including the gob smacking Telnet one in Solaris 10 - proff Sun's programmer's need a kicking.
Windows, due to its popularity, has had to become secure, and probably is the most secure of the popular OS's. Though any OS is only as secure as the person using it.