There are MANY reasons people dislike Unity..
The first haters disliked the fact that Ubuntu broke away from Gnome 3 shell development to create their own DE (some people always bitch at a fork), some thinking too many DEs' is a bad thing, others just thinking Ubuntu's leaving slowed GS development.
Second group of haters disliked Unity because it was out before GS, so was the first of the "new metaphor" DE's that people could hate .. they tended to dislike GS too, but less because they'd already spent their hate on Unity (though beleive me they hated GS too, at least at first)
There's the "it's not configurable (so not very Linuxy)" and "it's too much like OS X", and "dumbed down like Windows" and "going too commercial" haters
Then there's those that hate it because it's "in vogue" to hate it (half of which probably have little experience of it) or to hate Ubuntu in general, or only hate it so they can "big up" their distro of choice, or just look for any reason to hate the perceived "leader".
Then there's the group I belong to, that simply hate the whole "new metaphor" thing, both Unity and GS .. at least Gnome 3 can easily be re-shelled like Mint have done with Cinnamon .. Unity on the other hand is kinda like Windows desktop, you get what you're given which is a real pain if you think what you're given SUX

Personally I don't get people who say Windows "Metro UI" sucks, but then use Unity or GS .. they are all attempts to "unify" the mobile and desktop UI's, and they all fail miserably because it never needed unifying .. IMHO the "
desktop metaphor" was pretty much perfected a long time ago and doesn't need to change to accommodate "mobile", I know Microsoft/Canonical/Gnome think otherwise but people ARE smart enough to master 2 different UI's for 2 different devices

Mark my words .. the "new metaphor" DE's will all fail in the long run ..
forcing a mobile UI onto the desktop PC (as opposed to giving you an option to switch between the two .. Microsoft actually did that BETTER than Unity or GS), and bloating up mobile UI with features for desktop users is simply ridiculous.
(what's next .. a full on version of windows 10 / Ubuntu 16.04 on your router / calculator / mp3 player)
OK, rant over
