I doubt that anyone is saying 'OMFG GET 10001kW' for a laugh here. I am a regularly gigging bassplayer and can only tell you of my own experiences, which are that I would personally recommend 300w and above for peace of mind. You MIGHT be able to get away with half of that, but driving solid state amplifiers hard all night is not a generally great idea.
As someone said above, efficiency is king. I sound far louder with an Eden 4x10 and a 300w GK head now, than I did with a 4x10 AND a 15' from a 600w head. This is because the Eden is incredibly efficient. There are times I still wish I had the 15, not for the volume but the uber-lows, but then I get over it and play some music.
The same is also true for power amps - sadly some amps just seem louder, regardless of wattage. Anyone that has owned an old Peavey will know what I am talking about here. Likewise, some people have been dissapointed by the massive wattages claimed by some digital amps (i'm thinking Ashdown Superfly and Little Giant here).
It's not always about having what will just do it and scrape by, having some power in reserve is a seriously good thing.
Some people may say 'you never need that much power on stage', however it's my experience that not all bands and venues have a PA capable of having bass guitar through it (and the monitor mix), nor would always want it. Everyone's setup is likely to be different, so it makes sense to go for something, if practical, that will exceed your requirements, not just meet them.
* Edited to attempt to make me sound less of a prat *
Cheers.
James