Quorum? There is no voting for a soft fork. Miners either upgrade their client or do not. In a soft fork, all old nodes can see the new blocks, can still have transactions sent to addresses that they recognize, and can spend their existing coins.
Yes there is.. Soft-forks only reach super-majority (and the feature switched on) if the block versions produced by miners reaches reach a number indicating the change. 95% of 1250 concurrent blocks is the exact mechanism - miners have full choice over what software they run.
This 95% rule is also a rule that can be changed at will, nothing enforce it. If I'm a large mining pool, I will trigger it with even one block and start to produce new blocks immediately
It seems the last defense is exchanges, if exchanges do not upgrade to new software, then all the coins from those new blocks will worth nothing and become useless. You can setup a new exchange for exchanging those new coins, then most possibly they will become some kind of alt-coin exchange with neglectable USD value/volume
I think this is the biggest weakness of bitcoin, e.g. no systematic way to secure the rules from being modified, thus make the software itself a single point of failure
The only measurement we have today is a FOMC like commiter's group, and there is no clear guidance in their decision making, and Gmaxwell has already resigned