Technically, Vista was fine, but MS fudged bigtime on the business side.
There's a lot of people saying that Vista was buggy, which is technically incorrect. The OS itself was fairly stable, in some ways, more than XP. But, Vista changed the driver model. Why? because XP is hopelessly riddled with security holes on that front. Remember, XP came over 10 years ago, in a much more innocent time.
Clearly, the change was needed, so where did it go wrong? OEM support. MS went back and forth with the release date so much -whilst changing how the whole thing worked- that manufacturers gave up on trying to create drivers for an even changing platform. As a result, drivers at launch were rush jobs. Most dev teams didn't really get the new model. Hence all that incompatibility and instability, it wasn't really Vista's fault, it was the drivers. This got way better as OEMS got the hang of it, but by then the perception was set, and it was a lost battle.
With Vista, Microsoft was between a rock and a hard place. Keeping the -non existant- XP security model was not an option. But adding security would break lots of poorly designed programs. Seriously, tell me why a freaking game needs to run as the Admin.
Another way Microsoft dropped the ball was the "Vista Compatible Program". In trying to please hardware manufacturers the program allowed the approval stamp on machines that could barely run the thing.
And yes, vista uses more memory and its slower than XP. Of course it does! Its doing way more. Hey, I don't see anybody winging about XP using more memory than DOS.
Of course, at the end of the day, all the technical prowess of Vista is meaningless if the experience sucks, which it did.