I think this mode is awesome the way it is. I think the balance of jobs against each other is very good. Casters shine more than in Secure mode, but still get stomped heavily by melee if they aren't careful. I think the only real flaw that Slaughter shines light onto is the tab targeting system. If there was a better way to pick out a specific target, this mode would be a lot better/smoother. A lot of the reason you have a hard time dealing with people who go into a pack is the targeting. You see a weak guy, and you're like, I WANT TO KILL HIM! Well, 30 tabs later, you might be targeting him. Good thing he got healed already.

I'm not sure exactly what the solution is. You can have someone put marks on them and then target them based on that mark, but while I have macros for that, I doubt it's exactly universal. It would be nice to have an "enemy party list" or something (or to have more freeform targeting, but obviously the whole game engine isn't going to change). This is part of why ground targeted AoEs like Cometeor are so strong - because you don't have to bother targeting much of anyone. Just bind/sleep the crowd then lulz right in there. If you had more ground targeted abilities, you'd be complaining about those too.