MySQL always had crappy documentation. SUN has great documentation in comparison, see Solaris and Java docs. The Linux doc is the source code.

SUN has extensive track record of providing Enterprise support, for those who want it. It will cost, though.

MySQL was beginning to look for alternative ways to get money, for instance they quit offering the latest binaries for download. Why? What would next step be? Charge for the binaries? And then? Fork off an "enterprise variant" and close the source when they have great many customers? With SUN all this wont be a problem anymore, because of SUN's total commitment to open source all their high technology, Java, Solaris, ZFS, Dtrace, Zones, etc. SUN knows open source. 

We now get stable product with some excellent support, provided SUN doesnt go bankrupt.

One of the better moves by Sun in a long time. Probably even better then Open Sol.