Now for the negatives:
Collections are cumbersone. Maybe I haven't found it but I haven't seen a way to manage titles in bulk. Adding a book to a collection takes too many clicks. Also, I haven't seen how I rename a collection. I need to spend a little time with the manual me thinks.
Wikipedia formatting isn't as good as it was in K1. When I searched Wikipedia before, it came up nicely formatted for the Kindle. Maybe it was using the mobile version. Now it comes up in the browser looking like the full web page and I have to manually switch it to article mode. If I click on a link in a Wikipedia article, I have to switch to article mode again. It's slightly annoying.
If you are migrating from a K2, you can import any existing Collections from it (an option you'll find in 'Archived Items' on the new one), and any Amazon content you pull in from Archived Items will automagically categorize itself in the same collections. You are on your own with 'personal' content and Samples, though. And it is onerous if you have a couple of hundred items that you need to put into one or more collections. Note you can start with an item and add it to 1 or more collections, or start with a collection and go through all of the items to find the ones you want to include in it. But it's still a chore, and some people would prefer a collection scheme based on file system so you could use a computer to drag files around instead. That has its limitations as well though.
rename is easy: find the collection on your home screen, click Right on 5-way, select 'rename collection'.
K1/K2 used 'en.m.wikipedia.org/wiki?search=searchterm' for wikipedia searches launched from the search bar. So that's the 'mobile' interface to wikipedia. I guess they felt confident enough about the new browser to just use the regular version for this purpose, and in any case there's no provision for changing the behavior short of hacking.
I typed in the 'm' URL and it certainly loads much more quickly. But it also doesn't load the entire article, just the summary. You have to click on buttons to bring up additional sections, or on a link at the bottom of the article to load the regular interface with everything shown. Since it is going to be a matter of personal preference as to which of these behaviors is 'best', amazon had to make a choice and risk annoying everyone who would have chosen otherwise. It's not core functionality so a preference setting just for this would be overkill.
Maybe it is not too hard to hack it, though, to restore K2 behavior: just find the 'en.wikipedia.org' string, wherever they keep it, and change it back to what it was on K2. You might post on the Kindle Developer forum here to see if anyone can figure out how to do it.