I think the end results are all the same is what the OP is trying to get at. Whether you play monk, lance, etc. You can have totally different play styles, totally different tactics, totally different rotations - but at the end of the day, guess what? Everyone doing the same damage, its just different ways of pushing buttons to get there (assuming everyone is evenly geared).

In order to vary this you have to have certain extremes, like for instance, a Tank class that can soak up incredible burst damage but takes more damage from smaller hits, a tank class like this can be useful in boss battles, but will need alot of attention during trash mobs. A DPS class that does incredible burst damage but has practically no sustained damage.This style of DPS would be useful in titan Heart phase and adds phase, but would struggle for the rest of the fight - which then leaves the pre-made group with options: should we bring that dps because we struggle with heart phase? or ditch it cause we need more sustained dps? Those are options and choices that groups would have to discuss and agree upon. But then we run into issues with DF and queing, and getting 4 of those DPS in a group, which could cause some imbalance issues.

The more you start thinking about it and break it down, the more you realize that in order to have more variety the whole combat system and DF mechanics will have to be severely tweaked, I suppose its possible but doubtful this game will last long enough down the road for them to make those types of huge changes.