There is a very specific security threat when a single entity controls 50% of the networks hashing power.
50% isn't actually a hard limit. It is possible to do the same attack with 40% or even less, it will just succeed less often.
There are more problems than simply forking the block chain and double spending. 50% is a hard limit for being given control of parts of the block transactions. Leading to a whole can of worms in the grand scheme of bitcoin. The idea is that the less opportunity there is for malicious and potentially devastating problems with the block chain the more trust there can be in the idea of bitcoin as a viable tool.
Approaching 40-50+% lends itself to being far more vulnerable to all kinds of faith shattering events.
Well, Bitcoins.lc has NO fee. So there is a good option.
But, when about 10% of people move from deepbit to another 0% fee pool, then it will fix current situation, won't it?
The more the better, but if 10% of Tycho's users moved out that would definitely grant a large margin of error. The problem is, what is 10% currently? He has about 2.2THash of power, so that's 220GHash. If you assume each user as having 850MHash of power (which is more than most actually have) that's about 250 people.
There aren't even half of 250 replies in this thread. It's a bigger feat to accomplish than you might think.