Sorry for the delayed reply.

Ultimately, with a pool, he who controls the pool controls the processing power.  Yes, people can leave and join freely, but the incentive for people to group together and hand over control of their processing power to a trusted third party is a threat which is at odds with the idea of decentralisation.  It would be far better if anyone could plug into the network and retain full control of their mining activities, receiving a constant trickle of income directly.

And this is what I mean: what I have described defeats the whole genius solution to the decentralised ledger; it requires something totally different.

PS: I agree that massive farms are an even bigger threat.

I've spent the last year becoming comfortable with Bitcoin and associated technical and economic issues.  The greatest flaw in the system seems to be that it aims to be decentralised, and yet the incentive to mine encourages pooling.  The only reason I have not personally invested money into Bitcoin is because this incentive to pool mining resources seems to be a catastrophe waiting to happen.

I have been surprised not to find any discussions specifically about finding an alternative system which would not encourage pooling.  It's having to solve the problem of a decentralised ledger all over again but, never-the-less, it would be the most important advancement to make.  I suspect it is the only possible way an altcoin could steal Bitcoin's thunder.