What exactly has been changed/removed in Vista to get to 7

"It has a new name, and three years worth of Vista's self-improvement.

Windows 7 isn't much better than Vista. It's better, certainly, but not massively second-coming-of-christ sort of better. Anybody who truly didn't like Vista for good reasons (and they were there) probably won't find much to love in 7 either, but for those who just disliked vista because, well, it's vista (See: Mojave), Windows 7 provides a fast, solid OS that they don't have to deny themselves.

As an example of something improved, the window drawing code is slightly better, giving everything the appearance of greater speed."

"Windows 7 has big changes in the graphics subsystem. In vista some graphics operations are done by the CPU, some by the GPU. This means vista must keep a bitmap of each window in main memory, as well as a bitmap of each window in video memory.

In 7 much more is done by the GPU which improves graphics speed and also there is no need to keep a bitmap of each window in main memory. If you have lots of windows open this saves a lot of memory. Also GDI+ is now multi-threaded and more than one app can use GDI at once.

More details here: http://arstechnica.com/microsoft/reviews/2009/10/windows-7-the-review.ars/8

Also 7 and server 2008 R2 have a completely re-written scheduler which quite likely is leading to some of windows 7's perceved quicker performance.
More about that here: channel9.msdn.com/shows/Going+Deep/Arun-Kishan-Farewell-to-the-Windows-Kernel-Dispatcher-Lock/

(I'd make that a proper hyperlink but apparently I can't because I'm a new user :("