For me it was the video feeds and click through healing of ISBoxer - what a staggeringly, amazing product that is!

Before, in WoW, I needed octopus arms to heal my mixed group - with a variety of function keys, alt-function keys, ctrl-alt functions keys etc, to do different sized heals on different members of the group.
In big/stressful fights, it was easy to get it wrong and lose chars/wipe.

When I saw the posts showing how you could combine video feeds, clicking through and and in-game addon like clique (IIRC) to allow 'proper' healing of the whole group, it changed my mixed team into a dungeon stomping team of death!