High powered campaigns are always a lot more difficult to get right to balance between keeping the story compelling without going overboard with "epicness", much more so than low key ones. I would not say it was decline yet, even though it could have done better if they would have structured it so that you have to go through some wilderness areas before reaching places outside of Athkatla for example.
The romances were not particularly good either but there were some glimpses with Viconia for example which were good, like getting her away from the Drow ways in ToB and changing her to true neutral alignment, she also had the most ambiguous ending, wether you chose godhood or staying mortal, which is a plus in my book, not everyone gets a complete happy ending.
Ultimately though it does end up with you playing psychologist more than an actual romance though.
Also BG 2 has my favorite NPC in fantasy cRPGs, Jan Jansen, who is not a one dimensional boring joke like Minsc but had some actual genuine depth underneath all the silly stories and jokes imho.
BG2 is a pretty good RPG due to high quality content, but you can trace the seeds of decline (Bioware-style decline) in it. If you think about all the things that make later Bioware games annoying, most of them appear in their early form in BG2 (and not in BG1). The hub structure that everyone hates (a hub and 4-5 areas around it) was first explored in BG2 which although not as formulaic as their later efforts, did have hubs that you went out from as opposed to the more organic continuous structure of BG1. The forced epic content (a leech here, a dragon there, a spaceship over yonder, some uberloot in every drawer, epic spells, etc). The telegraphed main plot, that you pretty much knew everything about since the intro or shortly afterwards. I feel like it's vastly overrated, although it is a pretty good all around RPG.
Knowing the goal is not necessarily a minus for me. Just think of System Shock 2, everyone who played SS 1 knew it would end up with you going up against Shodan. The way is usually far more important than the goal imho.
I am not sure if you can call Athkatla a hub like the camp in DA:O. In comparison to Denerim (?) and the camp, Athkatla was huge and also felt like it was not constructed around YOU. They did not so much change the formula as they just reversed the flow from BG 1, where reaching Baldur's Gate as the big city came at the end, while here you start in the big city, that is at least how I see it.
As mentioned before, nothing is really placed conviently so that can easily reach it in mere seconds. Also you could actually fail with party composition and romances, unlike DA:O where you could just continue to give presents until you are at +100 approval, a sytem so mindboggelingly retarded it defies comprehension to me.
Companions you would not take with you would also not all stay conviniently in one place, they would stay in more or less logical loctations, albeit that they would wait forever is a bit silly but can be overlooked.
Also dying was prevalent and could be permanent, with no chance of revival. DA:O? Can you even die in that game?
The magic system, despite it's flaws and potentials for abuse, which you could avoid with very little effort by the way, was also superior in most ways. Spell combos were nice, but in the end you would do the same combo over and over. Meanwhile you would, or at least I did, change my sequencers and contingencies on a regular basis, also used some rather odd spells like Hold Undead on my Wizard PC for example.
What annoyed me most was the removal of actual wilderness exploration, so despite being the bigger game it felt kinda smaller than BG 1, and also it was too easy to rest, should have been nigh impossible in enemy dungeons. Heck with a simple trick like having some reinforcments at the entrance back you could combine this to make you think really hard wether you use magic like candy or not.