Liked best about SU (these days):
Easy sharing of sites and quick messaging.
In other words the toolbar, as a utility, is the focus of activity for me now because SU’s other features are compromised in so many ways
Reasons to retain my account:
Three years plus worth of links, reviews and jottings — plus the ability to contact people and send and receive pages.
I am still on SU because I have a history there, not because of what it offers now
Most missed:
Visual editor, dropped in V4 (made up for by Onyxstone’s fantastic SUEditor, which however limits you to Firefox, and is not supported by SU)
Drift of general change that alienated me:
Thumbings seen as more relevant than reviews
And what got my back up over a period of time:
The admin policing that became part of the culture as SU yielded to “family values” and SEOs
Biggest disappointment in recent times:
Lack of support — the “customer service” staff appear to either not know what’s going on or follow a tight script, and they often miss the point of complaints raised in the forums, or avoid addressing them

I have a personal experience of this, btw, in that a fleshy av I used for about three weeks recently made visits to my page rocket.
Some stumblers use this ploy intentionally of course but I did not, and the many men who contacted me were oblivious to the intended satire (it was a fat version of the pop star Shakira who I didn’t see as sexy!).
_______________
Now what I really love about Tumblr …
The whole set-up for bloggers encourages and fosters creativity (never mind the fact that most Tumblrs use the tools only in a very basic way)
There is support for seven different types of posting according to the material you have, you can upload files straight from your computer, and the “Theme Garden” contains an amazing array of 310 supported page designs. Most of those are customisable, and if you’re savvy enough you can even adapt them to your own needs or make your own. This excited me from the start, and still does!
And the other huge bonus for me is that there is no interference from above (see comments on SU policing, above).
________
My only frustration after 3-4 months’ use is that I sometimes find the different voices in some of the reblogging threads hard to distinguish. It is not a big deal though, and you can always look up the hyperlinked names to get a handle on people (or move on to the next item).
A large proportion of postings to Tumblr are simply mindless rebloggings and/or MySpace-type “noise” (and I’m not talking about the music), that criticism is valid; but if you dig, search and can attract people to your own pages with something you post, it becomes much more rewarding.
As a result of following trails from the search box and subscribing to particular tags, I have found more good blogs than I can keep up with and am excited to log in for a session. (I last remember feeling that way about SU a couple of years ago.)
Tumblelogs I love include hard news, original writing, special subject areas and the just plain quirky.
There is a small element of commercialism — if you search on anything that can be thought of as a commodity you’ll find it being offered for sale.
However, unless the “Ask” feature becomes problematic, there is no spam problem and that is another huge bonus.
_______________________
If you have a different point of view on either of these networks or something to say about “social media” in general, please feel free to submit it to Team Whiner.