"today most SSD drives internally act as if they are a raid set"
Maybe. But SSD drives act, externally, as 1 drive. A single drive with excellent seek time and a sometimes very good data rate. If the drive can do parallelization of a single stream of data that means the FS and block i/o layers don't have to be so fancy. +1 for EXT4.
Regarding multiple threads writing... do you really think media player streaming and word processor saving state is going to require a half gig a second via 5+ threads for extended periods in the near future? If so, I think I'll go dust off my TV and typewriter. +1 for Magnavox and Royal.