I have recently migrated from v2.2.1 which I installed following a recommendation a couple years ago, to 2.0.4 with Sequential download and DHT patches, and µTP, LPD and rate_control disabled. The process went well, the size of my resume.dat and settings.dat files decreased because some anti-virus related settings were removed, and "maxavgdl" bloating did not occur anymore. The appearance of the icons also improved (no more flat Metro look with oversized icons).
I have been annoyed by the changed beahvior of the "Set Download Location" in 2.2.1, where the torrent name was automatically added to the path as a subfolder if it didn't match the chosen directory. I felt a need to google up the meaning of the Yes/No/Cancel prompt several times, because it's just not intuitive. After I accidentally moved some files, which I didn't expect to happen during "setting" of the path, I made a decision to downgrade migrate for this reason alone.
From version 2.2.1 onwards, the program appears to change the language in which it addresses the user on the UI, perhaps to target a different demography. The function of some dialogs is now less clear than it was before. In µT 3 the definitions of path and torrent name were further confused in the Add Torrent dialog. Then, if I choose to delete a torrent, I dialog pops up asking for a comment to "fellow users", if I tick the checkbox "Don't show this dialog", this doesn't just disable the silly comment prompt (the new dialog) but the confirmation entirely. I have tried other 3.x versions, which now launch internet explorer pages to show "plus" and "devices", on the status bar there are links to twitter, which are messaging systems and nothing whatsoever to do with µTorrent.
After perusing the changelog, I got an impression that all the bugs that are being fixed result from incomplete superficial features to do do with autoupdates, devices, plus, offers, ads, dna, features which should never be there in the first place.
In short, changed behavior of basic operations, which by now I have already developed muscle memory for, as well as decreased stability because of the presence of additional parts of the program, are my reasons.
I am running 1.6.1 as my main client, which works very well at high speed with most private trackers, most of the time. But some new functionality added by the new developer team is occasionally useful, such as: relocation of individual files, source port ranges for advanced routing, magnet links, functional and fast webui, scheduler state for seeding only.
Potentially useful improvements: IPv6, uTP.
The need for these is kinda a problem created by modern networks and new versions of µT in the wild. I've encountered peers which had the bt.tcp_rate_control enabled, and would upload very slowly until I touched them by UDP. I have tried to use UDP in several versions, on both ethernet and wifi connections, including in BTSync (where there is no option not to use it), and it was always much slower. I suppose it was tuned for modems, so maybe it works for some people.
On a properly configured OS, all the new settings for port forwarding, socket buffer, and new protocols aren't needed.
Of the top of my head, the only function I miss from 3.x is the creation of torrents with large piece sizes for modern data sets. I have a 3.x for this job here.