There are in fact quite a few threads talking about what they dislike. They might not be titled as such, but there are plenty out there.
One thing I wish people would consider it that WP7 phones aren't trying to be iOS or Androids. They have a different way of doing many things although some little features are missing. I've only been on WP7 for a couple weeks. Although there are some things that need more attention, the more and more I use it, i find little easter eggs that explain why they chose to go the route they did and sometime it ends up being better in my opinion. Humans are creatures of habit. So if you've owned an iphone or an android for the past few years AND aren't really good with change, then there might be a few things that are undocumented that you may not like. WP7 phones have many of the basics covered when it comes to obtaining information from different sources all into one general area. In my opinion, I think the WP7 mail is one of the best STOCK email clients out there. That just my opinion though.
I like the fact that people from different OS are at least trying it out. If they don't like it and it didn't keep their attention, then that's cool. Not everyone has to like everything new. But no one could honestly say that WP7 is horrible. Especially when you consider the hardware they had to work with. I think other OS developers will wake up a tad when Apollo phones come out. Like others, they will pick and choose the good things from the Metro UI and copy it. Maybe even make it better. But that competition for you. I'm just glad that we all get to benefit from each of the companies being competitors.