My main reason is that it's a closed centralized service. I suppose somebody will say "it's not closed, it has an open API" and that misses the point: it's closed in the sense that you or I can't start a Twitter service and integrate it on equal terms with the original Twitter. Email / SMTP provided us a successful model and XMPP followed later to show how to do it for real-time messaging. It would have been easy to build Twitter on top of these standards, or to extend them in open ways if necessary. Instead Twitter is a giant step backwards into a time when technology and services were controlled by single vendors. We grew out of that and I thought we would never go back. I'm horrified that the tech digerati are all enthusiastically embracing a headlong rush back into it. I can't understand why nobody seems to care - it's like the enthusiastic embrace of the Iraq war by the media - there's hardly a critical voice to be heard, and yet I can see it will only be a short space of time before people are all realizing the cage they have built for themselves and asking what everyone was thinking.