1. User-space scheduling. It would be nice if a process could have better control on the priority of each of its threads. For example, on a web service where multiple users are active, it is often necessary to give each user his/her share of the cpu. Right now this is rather difficult to do in a fair way, since multiple threads may belong to the same user.
If normal priorities aren't sufficient, you can setup cgroups.
3. "Nice" for bandwidth.
For IO, ionice? Or, again, cgroups allows fair sharing IO and network BW, IIRC.
4. "Select" or "poll" with access to inter-thread synchronization structures. Select and poll are system calls which act mainly on file-descriptors. However, sometimes you'd like to wait also on a mutex or semaphore. Some support for this would be great.
Isn't this what pthreads condition variables are for? Or can you explain what you want in more detail?