Well... it didn't suffer from the hacker fest of d1... that I'm aware of anyways. It's certainly not on the same level of bugs and cash shop as Ghost recon.
The biggest problem was the loot, on multiple levels, which they've fixed on multiple levels last patch. Last night I was working on unlocking the technician spec and had to run through the invaded missions to get the arena open for a normal run, plus a capital hill run, and probably got like 30 set pieces, a couple perfect talent pieces, and the exotic shotgun from random and end mission drops.
They introduced a lot more random stats and talents to gear than d1, plus colored gear mods, plus talents requiring colors, plus limited slots for brands, plus set requiring 6 pieces, that made the system a clusterfuck to deal with. Now with the brand being all slots, stats being able to replace one entirely for anything else, and sets back down to 4 makes the system a lot better to create builds around (although the sets in d1 were way better so far). Only took them 6 months to get their vindictive heads (gun mod system they obviously hated that players hated it) out of their asses and finally fixed stuff.
Crafting is better, recal is a little better organized and the swap between any color is a good change, I can understand why they wouldn't want to bring back optimization back this early into the game's life, it's a looter shooter and loot is the hamster wheel they need you to be on all the time, if you end up with the perfect gear in a lot less time, some motivation to play the game is going to be lost. If you're not primarily grinding for gear in these games what else are you doing? doing fluff achievements or getting the lowest clear times?
Content wise they've released new missions, expeditions which is missions on like a 3 week rotation timer and suppose to have mastery to build around? not a fan of this, they've had more open world events to do than d1 ever did, next raid delayed to next year (I don't care). But they haven't released anything extra like survival or underground or resistance modes yet, all of which were like year1 year2 releases anyways and probably plan on bringing back at some point.
Was it worth being a new game than an expansion to d1? depends I guess, raids replaced incursions, the open world is a different season and supposedly 20% bigger than the ny area, and was set up differently with much more activities to do from the get go, missions pulling double duty with invaded versions, hvt turned into bounties, the new specializations give you a different way to play but are basically talent trees. I would guess there's enough there to be a new game than an expansion. They just tried to experiment with too many changes to the entire loot system (basically add more rng layers to make it take longer to get perfect gear) and failed at it.