Make something people want. Either people want it or they don't, but I say judge a startup by that and not just technical innovation.
So who wants Yammer? (apart from the VCs) Do you know anyone who is actually using Yammer?
If the Twitter dev team were able to give me everything I wanted, then I'd want the ability to start a private group with people, where updates into that group are protected from my public timeline.
I'd also like the ability to add custom tabs onto my twitter UI so that I could get instant context of any conversation happening within my group, while maintaining the ability for these messages to get forwarded to my mobile device if I wanted.
I would pay for a service like that, absolutely.