Most of the current ways things are done in MMOs are to overcome obstacles involving latency. Your ideas on positioning (intercepting attackers, physically blocking attacks, etc...) are of course good ideas, but they tend to fall apart in a simulation where oft-times large latency is present, which is pretty much every MMO. Everyone who has played with huge ping times is familiar with rubber-banding and other occurrences, and these issues are only aggravated by the requirement of precise positioning. A raid wiping because they failed the script is one thing, but a raid wiping due to latency issues on the tank's part is a whole different ball of wax, one that annoys the piss out of the players when it happens. With tight timing and positional requirements plus latency, this type of wiping would likely become the rule rather than the occasional occurrence.
You make a good point, running to a positioning to block wouldn't really be feasible because of latency. However, you could have spells that put a static object on the ground that players can stand behind. There wouldn't be any problems with latency there, or instead of running in the traditional sense couldn't you cast a spell where you want to jump? As long as the location is pre-determined doesn't that fix the latency issues? If the reaction time is less than the latency the player should successfully block the spell.