Twitter in real 'real time'
Instead of seeing tweets in my personal Twitter feed updating ever minute or so, they appear in the very second they have been published by users I follow. For some this might sound like a minor feature, but in my eyes it is huge, because it's only with the new API that Twitter has become a real "real time service".
Since weekends are usually calm times on Twitter I look forward to the next hours and days to see how and if the real time feed changes my Twitter experience. But already now I enjoy just sitting in front of my screen and looking at the continuous stream of tweets, without me having to do anything. It's like TV, just much more interesting.