It's easy to enumerate the movie's failures, but getting to the root of what's wrong is a little harder. Bear with me.
First, there's the race fail. Much as I like Tilda Swinton, having her play the Ancient One is kind of a race fail. Except there's NO ONE in this movie who's Nepalese or Mongolian or even Asian, as far as I can tell. Even the token Asian-heritage team member is played by Benedict Wong, who's English. That said, Chiwetel Ejiofor gets far more screen time than Swinton and though he's a PoC he's not Asian either.
The movie fails the Bechdel test, badly. Rachel McAdams does an excellent job and I think she's pretty well-written in the script. She plays Palmer, Strange's occasional casual partner and someone who was in love with him at some point, but she's neither simpering nor putting up with his BS infinitely. She does fine on her own, but she is on her own.
The movie also fails the "interesting plot" test. Frankly, the most interesting thing in the movie is Dr Strange's Cloak of Levitation, which has its own personality and motivations. It's funny and useful and looks good doing its thing. But when your CGI magic object is holding more attention than your plot you have a serious problem. The villains are bog-standard and boring. Mads Mikkelsen is clearly suffering with that intense eye make-up but he does his best.
The moment that cued me into what's wrong with the movie comes about 3/4 of the way in when Strange complains to Mordo that he (Strange) has just been forced to kill someone. He says something about having taken an oath to preserve life and he doesn't want to sign up for a conflict in which he'd be expected to take more lives.
That seems to be a hugely important thing about his character and motivations and yet it comes out of nowhere. We spend a lot of time in the first half of the film exploring Strange's background and motivations - his ego, the fact that he spends money as fast as he gets it, etc. In the climactic scene between Strange and the Ancient One it is reinforced to us that Strange's motivations, and the idea that it's not all about him, are supremely important - they're what's holding him back she says. So how did something this big fall through the cracks?
Option 1 is that it's just bad script-writing. Maybe it was seen as a throw-away bit to try and give Strange some more sympathy because he's pretty damned unsympathetic most of the time. Given that the movie has three credited writers there might have been a lot of cracks for things to slip between. Option 2 is that it's something that was set up earlier and ended on the cutting room floor. IMDB tells me the movie comes in at 1 hour 55 minutes and it doesn't feel that long. It feels like it moves along reasonably well. I could easily see how an original cut would have been closer to 2:30 and they decided it needed to be sped up. Speeding it up would have been done by cutting more of the back-story, and likely some of the Strange-Palmer interactions.
Overall I didn't hate the movie and didn't feel like I wanted my money back, but they need to figure out what they're doing with this character and do that thing well in the next movie.