Melee jobs tend to have awkwardly long combos, and for max efficiency, you need to weave in buffs, abilities etc, at specific points, so it lines up to be able to get maximum burst damage when main FPS buffs come off CD, which can be 3min, so your combos are basically 3min long. Also you need to time it all with boss rotation so you don't use heavy hits as they become invulnerable etc.
So for reaper you'll get some dps buffs, and you want to align that with possession, without any of them being available and wasting time, waiting for the other.
Because of this you'll spend more time looking at your skill bar, than at the fight, missing mechanics, and die more often tanking is quite simple, cause you can spend more time looking at boss/cast bar.
You won't be able to easily solo lots of content, because you're squishy and no reliable self heal - though RDM can sustain themselves, so we'll see how reaper does.
Please look forward to it.

Forget about running dungeons at release, lv 70-80 - your only decent XP will be from FATES. At lv 80 you'll do MSQ, at 81 you'll be able to run Trust dungeons.