I hadn't heard that WAR would have collision detection (it must be in the whole game, right? can't only be in PvP, can it?), but Hardcore Casual did, and just like him, I'm very interested in the possibilities. He cites it as a problem of premades and PuGs-- premades will know much better how to deal with line-of-sight and positioning and formation problems, while PuGs will trip over themselves trying to get past someone else. Me, I see it more as a problem of pranks and griefing. In World of Warcraft, where Blizzard hasn't implemented collision detection, they're already having issues-- giving players the ability to block space in large numbers just opens up a whole world of griefing.
But as HC says, it's all in the implementation-- it may be that some things need line-of-sight, while others (spells and such) don't, and any blockage depends on just where the hitboxes are-- if characters can sneak past each other, or are able to push each other out of the way, then that would change any plans.
It'll be interesting to see how it all pans out, however. Collision detection is a minefield for developers, but can definitely lend itself to new types of gameplay. While most MMOs have avoided the issue entirely, we'll have to see if WAR is willing to hit the issue (so to speak) head-on.