First of all, I had had this on my to-read list for a very long time. I had read a lot about it and decided beforehand that it was likely to be one of the best of (many!) the YA dystopian novels on offer. So, in all fairness, I did go to it with quite high expectations.
I'm still a bit unsure of why I didn't love this. I thought our heroine Tally was a pretty good main character, not too Mary Sue-ish or vapid and I thought the world-building was well done and detailed enough that you felt you could picture it.
Unfortunately, the world-building for me was only well-written - meaning that while the author took some time to explain how things were they way they were, he never really convinced me as to its reality. I don't think a good dystopian novel needs a long, explanatory back story (one of my favourites is The Road, where you are given more or less nothing), but I need to feel the world - that it's real and not just someone's imaginings. Which of course it is :-), but I want to lose myself completely in the book's universe and I never really did.
I think the biggest problem was that I was never fully sold on the whole idea of becoming a "Pretty" - I was never convinced that it made sense for Tally (and society) to want it. I mean while the "new pretty" lifestyle would maybe be appealing to a raging-hormone teenager, I don't really buy that they would all e.g. be happy to never ride their awesome hoverboards anymore.
The action in the story was decent - Westerfeld managed to get quite a lot of action into a relatively short book ... and I particularly liked Tally's journey across the country/land to find the outcasts , but I think the main problem was that I never really cared that much for what was happening and by the time of the "big reveal" (the explanatory back-story), I think we had all seen it coming miles off - at least if you've ever read any dystopia/government control novels before.
So, I guess my biggest problem was that I never really managed to suspend my disbelief for the story and just emerge myself in it. Which is something I'm usually VERY good at and actually get teased for because I'll read anything as long as it takes me along for the ride! And Uglies was just too constructed and failed to make me believe the basic premise of the story - which will ruin even the best written action.