I have a lot of fun in Slaughter — that's interesting, because both modes are about zerging down an NPC object.
But in Slaughter:
- The battlefield is tight, claustrophobic, and dangerous.
 
- Enemy confrontation is inevitable and intense.
 
- In-between Drones, there is non-stop brawling.
 
- Drone spawns are predictable both in position and clock, so teams know where to condense to, and you can also anticipate and push to slaughter the enemy team and deny their Drones right before a pop.
 
- Drones spawn in mass quantity, so it's not effective to just zerg one point, the action is necessarily distributed.
 
- Drone distribution and tiny HP means you must watch your flanks to prevent opportunistic opponents from nibbling on your Drones before you're finished.
 
Slaughter feels exciting, and despite the NPC objects and deathmatch vibe, I still have a lot of incentive to use all my tools as a PLD, and my team is very much rewarded for me doing so. (Maybe. Hopefully. It seems so.)
Shatter, in contrast... it's like everyone's really exhausted and just wants it over with. I run up to Crystals, and even if I'm in the middle of a huge pack of Flames (sorry guys, ♥ Flames, I just want my queue to pop) or Mael, everyone just ignores me so they can keep DPSing the Crystal harder. And I don't blame them.