It can take a while to finish absolutely everything, including finding all the lore and such, depending on your playstyle I suppose. But once you reach endgame, it is a continuous grind until you reach your max Light (power) level, then you just play to get cosmetic rewards, I suppose.
Can't really comment on much end-game activity since I've barely been able to scratch it since it released on consoles, rip. But endgame is definitely where the story and content begins.
My favorites are mostly PvE group activities. Very satisfying downing difficult bosses and passing challenging obstacles and then dancing in triumph.
The DLC won't fix everything. The tradition is that Bungie says something, the community gains Hype™, the game comes out and the community enjoys it but after a month when there isn't enough new content to satisfy them they begin to bicker and complain about all of the game's shortcomings, until the next expansion. It's been this way since D1, and probably could've been made better if D2 was just an expansion of the first one, but I digress. Right now, Bungie's just said that going forward, they plan on communicating better with everyone, which they've said since the first issues of D1 so what can ya do.
Still, this game is very enjoyable if you play it casually or if you play it more hard core with raids and stuff. You'll only be disappointed if you let yourself, imo. Also, Rahool will disappoint you, too.