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C/C++: Usually faster, standard code is machine-independent but most non-trivial apps will need to be ported.
Java: Slooooowww, but the `compiled' code can be ported, rather than just the source....

>>
...if NASA used Java they'd never get a response from their robots on Mars -
>> they'd still be loading and interpreting the code. I'm sure they use C and assembly.
Sorry Bubba,...

Well, writing a decent forum is time-consuming, so most people don't bother.
There is more than forums in web programming: dynamic pages, user-dependant permissions...