Whew... after nearly 200 hours - finally finished W3 along with all the DLCs. 
What CDPR had done here was an incredible achievement, so... much... content. And so much of it fleshed out. The kudos they get for their secondary quests are well-earned, no question. BioWare should definitely take notes here. And while I was a little more mixed in my opinion of the main game, the DLCs really helped elevate my overall opinion of it. I felt they were the most interesting/strongest parts of the game.
Main Plot: There were a number interesting characters and situations along the way, marred only by the tediously long questline in finding Ciri. There's one point, where Geralt's standing in a room trying to recruit street-promoters to get the word out for a play he helped write, cast and will have to act in, in order to draw in Dudu, to ask him where Dandelion is, and then rescue him in order to ask him where Ciri is... I'm going, What in the actual @$%# am I even doing here?! I could scour, on foot, the entire @#$%-ing country in the time it's taking me to do this @#$%!
Taken as individual quests, they're pretty interesting (and varied), but strung together in this nigh-never-ending chain of events in a game of "Where in the world is Ciri Sandiago", it nearly lost me a couple of times.
But when we finally get to the Skellige & Wild Hunt questlines, and finally finding Ciri, it starts picking up again. Playing Ciri and watching her evolution was fun, though I was more than a little ticked that we couldn't gear her up as we wanted. And because Jo Wyatt had voiced her, all I could hear was Hawke. 
I think the most interesting thing about the Witcher's quests were the sudden left turns it would take. A string of predictable quests, then-suddenly-I'm-exploring-other-worlds. A string of predictable quests, then suddenly I'm in a painting. A string of predictable quests, then suddenly I'm in a fairy tale land. A string of predictable quests, then suddenly I'm talking to my horse... and she's talking back... and is actually pretty interesting too. That's the sort of thing I wouldn't mind BioWare indulging in more.
Characters: There were a number of interesting characters, though very few we really get a chance to know well (like we would our companions in BioWare game). I liked the ones we could have more creative banter with - Dijkstra, the College of Sorceresses, Yennifer, O'Dimm, Vlodimir, even if we wound up on the losing end of those conversations more often than not.
CDPR do love their "grey" characters, which adds depth to those that might otherwise have been one-note characters like the Baron.
But since nearly everyone is at least part self-serving @$%#, I felt little empathy for most of them. This is something BioWare should be wary of not falling into the same trap. Other than Ciri and maybe a couple of others like Triss or Priscilla, I felt no attachment to the characters. Whether they lived or died didn't matter and I found myself just taking the path of the greatest reward vs making emotional decisions. BioWare games I feel are better here at eliciting an emotional response, and making emotional decisions over just pragmatic ones.
Game World: The scope of this game is even greater than I was expecting, and I knew going in it would be huge. Kudos to CDPR for putting all that effort in. Cities especially, I can't even imagine the man-hours that went into creating them. Day/Night cycles, dynamic weather, fog, torch-lit streets, etc. Hopefully we'll those first two make it into future DA games. Which makes me all the sadder to say... it's... all kinda boring (except for maybe Beauclair). I think the game engine might be part of the issue with the flat-ish lighting and desaturated feel to the game. But the locales they created are... just not that interesting and constantly dreary weather takes it toll as well (Toussaint being a much-needed breath of fresh air here).
In trying to faithfully recreate Middle Ages Europe - landscape, cities and all, there aren't many visually striking locations. With DAI, I think they took the right route here. Wide variety of environments, each visually striking. The Frostbite Engine really showing its chops here. I think BioWare's other strength here is that (IMO) they have a more interesting setting, one that I wanted to learn more about lore-wise, history and so on. BioWare and Bethesda are good at world-building in general.
Gameplay: There are things BioWare can learn from W3 here - combat on horseback, the races, swimming, climbing, sailing (which was a lot of fun). The fist-fights were so-so though. I wasn't a huge fan of Gwent, but I applaud the addition. I loved the parties in both HoS and B&W and the fun little things you could do during them. These little diversions oddly enough wind up being among the more memorable parts of the game. BioWare does this as well and hopefully they'll continue to do more of them.
Geralt's abilities/combat were... pretty good I'd say. Not great, but not bad. I've tried a number of different combat styles/ability mixes over the course of the game to keep it fresh. The action-y style was a nice change of pace from DAI's combat. Each has their strengths though. I don't think there's really anything DA could learn here, being such a different style, except for the mounted combat perhaps.
Overall, W3 is an incredible game with an astounding amount of effort put into it. And there are things CDPR and BioWare could certainly learn from one another.