- Message storage in absence
- Limit on amount of contacts
Probably for reasons of spamming and malmarketing you can follow up to 2000 people, maybe less in future. You may argue, that the service becomes pretty unuseful with so many subscriptions in place, anyway. Then again all those trend researchers require automatic analysis of pretty much every posting on the service.
- Problems due to heavy load
It notably ran into scalability issues (or rather, complete blackouts) during popular events.
- No interactive chat functions
@replies, #hashtags and the client API with its 100 calls an hour limit are all hacks compared to a true chat and messaging subsystem. And still you have no chatrooms, just the complete audience of your entire listenership.
Yet, every Twitterer would argue that microblogging isn't the same as chatting, which is true. There are subtle differences in the approach, which aren't typically covered by a chat system. The technological implications are very similar instead.
The @replies are increasingly being abused for spam. The mechanism that was supposed to give the fan a way to reach out for a VIP starts to fail as popularity rises. Hashtag spamming and direct message spamming are also quite annoying. Recently trojans are increasingly being offered by direct message.