That Swatting thing is ridiculous. Idiots.
Leveling a character up to twenty has been fun for me, for the most part. It's fun getting to play with the different abilities since I played my Warlock for so long. Shoulder charging Fallen and throwing knives in the Hive's face is a bit different to magic slapping them. I think it would be much more interesting if all three classes had a few abilities rather than just one main, melee and grenade, though.
It's when you get to 20 and the grind starts again that fun drains out of the game rapidly.
It's interesting that advancement is tied to gear, but I think they should expand the Light system in some way. Perhaps a way, maybe via a mission or exotic bounty type thing, where you can transfer the light of an item into your character themselves. So you could do a few things, like swapping that piece of gear for another with different stats/upgrades and leveling that to get more Light overall, or just for something you find more aesthetically pleasing. Maybe that would also help with so many high level players of the same class looking the same apart from shaders (and even then, there needs to be more shaders). It would also mean that going forward, people who don't or can't raid, would still be able to level up.