I voted Flashpoints - as this is where it is most needed. This is almost dead now and we need ppl to get groups for this more easy. I'd prefer x-realm coz i have the view that it is more important to get groups fast and efficiant then to stay whithin your own server. Bioware saya they wanna keep the server community by making groups inside the server, but i feel this only has limited effect on the server community. X-realam also helps grouping on unbalanced servers, so i feel its a much bigger pro then a con.
I do feel people need LFG tool for HC quests and Operations also.
Something simulare to the PvP tool would be awesome - you sign up for a HC FP and when your group is ready you get message. Though it is needed to be ways to kick players going afk or being unfriendly, and there needs to be a panalty for leaving.
Personaly i feel the LFG that eventualy came in WoW was very nice and well receaved by the community there, for worling well. After a lot of experiance they found out that giving rewards for roles that were rare gave more signups for this role, and that buffing "random" groups also motivated players to sign up for this.