There were a few issues with Vista. Not least of which is that it was too bloated for the PCs of the time. When Vista came along, PCs typically only had 2GB of RAM, sometimes only 1GB, plus they often only had single core CPUs. And Vista crawled along on such systems, compared with XP that ran pretty quick on the same hardware. Hence Vista gained a reputation for being slow.
Windows 7 is a similar size, but by the time it was released, 4GB was the norm, as were dual/quad core CPUs. Hence on the PCs of the time, Windows 7 ran pretty well and automatically gained a reputation of being quicker. Run Windows 7 on a single core system with 1-2GB of RAM and it'll also run sluggishly.
UAC was a good idea, but was quite annoying and popped up a bit too often. Windows 7 fixed this simply by toning down the prompts to areas where they were necessary. Of course, XP-style apps that expected admin rights and full write access to the whole hard drive definitely helped exacerbate the problem.
Lastly IMO, the main other issue of Vista was that it felt messy. 7 different shutdown options (with Sleep as the default - even on laptops). A default wallpaper that was a smorgasbord of too many colours, the side-bar that didn't do that much but which added more clutter. Windows 7 in comparison did a good job of streamlining and tidying the UI up.
Overall, I didn't personally like Vista. It was a bit too bloated for hardware of the time, the UI was messy, and it didn't offer much to the end user over XP, whereas Windows 7 introduced the new task bar, the ability to snap windows side-by-side, plus other genuinely useful tweaks.