I've been following NaCl, and it worries me that the compiled code only works on a particular version of Chrome. NaCl 0.5 only works with Chrome 14, 0.4 works with Chrome 13, etc.
Downtime comes from people. The more people involved, the more downtime you'll have.
Two weakest points of Java are:
1) Date/Time objects
as compared to C#/.Net -- having worked professionally in both. No langauge is going to fix these two.
(Though you can use Joda for date/time handling.)