The 140 limit is a bit annoying sometimes. I do feel it limits people's expression and as such i dont think Twitter is a good forum for detailed one to one conversation.
The trending topics is a total red herring, I never ever look at them, and i'm sure many other people dont either.
Real strengths for radicals;
the ability to delve into networks of interested individuals through a fairly small entry point. I mean, you only need to follow a few people on Twitter to then have access to all the people that they follow, and all that follow them. So from a couple of people you can suddenly be connected to a much much wider audience who's opinions you actually value.
Establishing counternarratives, calling out bullshit, providing links etc.. like was said above.
Harnessing the crowd, either for diligence (eg -Prince Charles travel expenses recently) or for sharing services - operating outside traditional consumer models.
Its speed, is perhaps its biggest asset. I dont have a phone with internets, but the ability to record and broadcast instantly worldwide is so powerful for activists - you only have to look at OWS and their use of Twitter/Ustream to rally support against police violence.
Dangers - the hive mind, which Mason might argue is a fantastic example of horizontalism in action, can sometimes descend into a baying mob. Groupthink can prevail too easily in that frenetic atmosphere.
Also, i agree with Virilio regarding speed, so while i think its one of Twitter's greatest assets, i also think that the acceleration of everything to the instant is generally a negative phenomenon.
Oh, and then there's centralisation, snooping, logging, prosecuting and persecuting that comes with Twitter too