Even without ads or promoted tweets, the network's a breeding ground for trolls and hatemongers of all kinds - the fact you only need an email address to set up an account makes it difficult to link back to a real person, while all tweets are public by default and it has an asymmetric relationship model (i.e. you can choose whose tweets you want to follow - they don't have to do anything on their part for you to see their content). Couple that with the sparsity of the interface and the brevity of the messages and it's very easy for both those who deliberately spread hate and those who say something that could be misconstrued as hateful (often in the latter case getting a lot of hate tweets in response).

The above features are both the network's biggest weakness and its biggest strength - you can tweet using your usual internet alias without worrying about the network blocking you for not using your "real" name, media commentators often hang around there to pick up an instant feel of the mood of participants, particularly in relation to current news / media events (it was the network that first introduced hashtags), while the limited range of things you can do with it makes for a relatively simple AI, so allowing code to tweet on your behalf (e.g. tweeting when you update a page on your website, breaking news events, weather details, and even more esoteric things such as the Radio 4 schedule and when Tower Bridge opens to allow ships through).