PS: The latter's appeal for me is that the technology is libre, but I don't want this to be the only determinant.
Not only is the technology, but the data is also open. The StatusNet people (BTW StatusNet is the software that powers Identi.ca) aims to build a system where switching from one social network to another is painless. You can find more information about the specification at oStatus. The idea behind this is, switching your social network should be just as easy as switching your ISP.
I have been using Identi.ca for about a month (you're right, I'm a newbie). The strongest impression I have on the site is the amount of openness. While Twitter seems to be hiding the #wikileaks tag, it is obvious on Identi.ca that the topic is "trending". There are also a lot of FOSS supporters who use Identi.ca (the two groups with most users on Identi.ca are !Ubuntu and !Linux).
Regarding features, the one feature on Identi.ca that is most different from Twitter is the group. When you join a group, you automatically receive all "tweets" that "mention" the group using the "!" syntax.
If you would like to read more, recently Why Identi.ca (and Status.net) matter in a Twitter world has been a popular post.