The login name problem
I have been vaguely considering getting a Twitter account or two for a while, but so far haven't done so. As before, the big stumbling block is that Twitter makes you pick a username and none of the ones that I find even vaguely attractive are still available. My usual login name is taken, as normal (it goes fast on many services). So is my last name (probably by a relative), my first name, a variant I sometimes use, a variant of my last name, and I don't feel like going on any more (it's too depressing). I can find only one vaguely appealing variant that isn't already taken (and I'm not going to say what it is).
Part of the problem is that people on Twitter use your username, which places a premium on short and memorable ones (especially given the size limit on tweets). But beyond that it's not just that Twitter requires me to pick a username per se, it's that it requires me to pick a public, more or less permanent identifier for myself. This is a fundamental problem because, as always, good names are hard. They are hard for people to come up with and there's only a limited supply of them.
(Twitter apparently allows you to change your username, but I suspect that that orphans your old Twitter URL and probably confuses people who knew you under the old name.)
Doing this is generally not actually necessary in most web services. Flickr, Facebook, and Google Plus (despite its serious flaws) all get this right; you can start using each service without creating such a permanent identifier. Oh, Flickr and Facebook have optional permanent IDs (and G+ may as well someday), but they really are optional; you can use the service for years (even as a paying customer) without having to commit yourself to one, and everything works fine. The most that happens is that the URL for your stuff is somewhat uglier that it could otherwise be.
(To be fair, all of these services make you give (or pick) your name and Flickr makes you pick a username. However, you probably already have a name you want to use and you can change all of this stuff if you want to. I've seen people rename themselves on Flickr all the time, not infrequently to add temporary status messages.)
The usual reason to force your users to pick login names is to generate URLs for them. However, Flickr shows that this isn't necessary; you can generate ugly URLs for now and let users improve them to nice URLs later when they make up their minds. Flickr even has convenient ways of referring to people who have not done so.
(To be fair, what differentiates Twitter from Flickr here is that Twitter wants people to be able to enter tweets as essentially plain text from outside itself; Flickr is content to require you to use its special markup to refer to other Flickr accounts.)