There is really nothing objective that is pointed to as to why it "sucks". I get it you don't like it. I imagine most people do, here is an XKCD comic about the situation your in http://xkcd.com/1172/
The post repeatedly mentions functionality that used to be one click away, but is now multiple clicks away. That is about as objective as measures come. You may not mind it, but you can't say it's just subjective.
The post also points out how UI controls shift relative positions based on the view being presented. One could define a metric that assesses the effect, and anybody calculating the metric would get the same result. Again, you may not mind controls jumping about as you work, but you can't say it's not objective.
I also use GMail mostly for personal emails, not professionally. I wonder if that's part of it. But, even then, I use Notes for professional emails, because I have to. The GMail changes work well for how I use it, but there's also the fact that I constantly compare it to Notes.
If you're writing a marketing e-mail with HTML or something longer, it's probably not great. If you like to use a lot of rich text formatting then it's probably not great, but for actual, pithy e-mail, in plain text, quickly, I think it works really well. Keeping your messages in the background to cross-reference things without opening 12 windows is awesome.
That said - I did have to search for a second to figure out how to edit the subject. A couple of quick tweaks or the addition of a "message settings" button for more admin-type tasks would make it just a bit better.
Everyone's a UI critic.
As it should be... everyone uses the UI for mass market products. Very few people admin the app or have the ability to access and analyze the source.