What it needs?
I see no signs of an API, something I found pretty much the day I tried twitter the first time, and which I think has given twitter a whole ecosystem of clients and sites revolving around its public timeline and users (IM clients, Phone clients, Web Clients, Statistics, Search, WordPress Integration, etc). A-Team you need an API ASAP.
Plurk needs an API, and probably alternate views, the whole PERT style visualization sucks, it makes it hard to read. But I guess it’s probably a decision to make themselves different from other competitors.
Also the Invite Friends feature, currently allows you to spam your friends, but it’d be more useful if they allowed you to enter your Twitter username and password and send the plurk invite via Twitter, at least you know all those people are more likely to try out this service than anyone else. It’d be so evil, but it’d work a lot better than email invites. Although… thinking about it, all you gotta do is send a twitter telling all your followers to try out plurk.com :p