Also, the female in The Departed played a significant role in the final payoff, but otherwise, she was pretty underdeveloped. She had a few card scenes, but the plotty nature of the movie gave away a "surprise" turn for her once it became fairly obvious that she didn't really have a part to play in the main plot. And sure enough, in the end, she is there to deliver the final zinger and then disappear. We NEVER KNOW what her true feelings are, whether she's for Leo or Matt, and we never know what she feels like about Matt turning on her, aside from putting the call in to Marky Mark. We know she must have hated him for his dishonesty, and that's it. She, the only female character in the story worth mentioning, is nothing but a PLOT DEVICE, and a lame and obvious one at that. Her character got about just as much development as the CITIZIN envelope that gave Matt Damon away (another plot device, and one that ended up being 100% inconsequential to the plot, wasting a nice little twist).
Those are the sorts of things that make me say the ending was a little too twisty for its own good. These things, in movies, are supposed to resemble nice little packages; the ending here almost got that, but there were little issues. The excuse "REAL LIFE DOESN'T MAKE NEAT LITTLE PACKAGES" will not hold up, so don't even try it--the whole movie made plays and references to symmetry throughout, so the lack of neatness in the ending is well, messy.