• The system counts paladins, druids and warriors as tanks, and shamans, paladins, priests and druids as healers. Now this system would work OK if you end up with a rogue, hunter, druid, shammy and mage, provided everyone is specced for their task. But if the druid is a moonkin and the shammy is dual wielding? That could be a problem.
• You can't look for more than three instances at a time. If someone is a new level 60, and would happily hit Dire Maul, Scholo, Strat or LBRS, they can't let this preference be known.
• Not enough people are using it. I know it has only been online for a couple days, but I have run into this problem myself. Some forum folks are claiming that the system itself is too difficult for people to use, which is keeping players away, but it seems pretty straightforward to me.
• The level restrictions seem a tad off. For example, some players were complaining about a level 40 being able to LFG for Mara.
• The lack of support for things like PVP raids.
• When you are on an alt, you can't search for groups for your main. You can't find people looking for Scholo if you are on your level 33.

Via WoWInsider