I think the idea is that competition triggers the incumbent to do better. Like how Google+'s circles finally forced Facebook to better model inter-personal relationships (at the time, only "friend" or "not friend").
Nobody knows what will result here but Twitter has been worsening of late so it's worth a shot IMHO.
Facebook lists existed for at least two years prior to Google+. Nobody used them because nobody wants to meticulously organize their friends into buckets and categorize their communications.
We have infrastructure. It's called AWS. The infrastructure of App.net is not revolutionary or even that interesting. Why do new social services need this infrastructure?
Nobody knows what will result here but Twitter has been worsening of late so it's worth a shot IMHO.