There's been a big problem recently with people mixing genres that don't mix. Let's take the most common offender: RPG and Strategy.
The rational seems to be "Similar people like these games, and often a single person likes both kinds of games, so putting the two together seems like a good idea." The problem you run into is that the fundamental principles of each genre are competing with each other. The fun of an RPG comes from accumulating more and more crap over time. An RPG where you don't gain anything isn't really an RPG anymore. Strategy titles want players to start as evenly as possible, so that some measure of player skill (multi-tasking, micromanagement, etc.) will let one player overcome the other. If what makes the difference is "how long have you been playing", then you have a strategy RPG, but it's not very fun and tends to eat itself because whoever has been playing longer almost always wins (barring terrible, unwinnable matchups that are equally unsatisfying).
Sometimes it's some particular gameplay element driving me nuts. I remembered a game named Savage that I played a demo for. One of the problems was that you earned points for getting kills, which you then used to get better equipment when you respawn. The problem with that was that you end up with somebody running around with bows and arrows shooting at each other, and one side tends to rack up a bunch of kills. The game is effectively over at that point, because even if you (with nothing but bows) manage to kill your adversary, they come back with kick-assed flamethrowers or electric guns that tear you apart because you can't afford them when you respawn (to be able to afford those required multiple kills). So you end up with the first skirmish deciding the match (at least at the time when I played it, which was early).
The "good" combinations of Strategy and RPG have been so-so. Warcraft 3 is one of the better meshes of this water-and-oil combination. For the single player experience, it leans on the RPG side more, with persistent characters that gain levels as you move further into the story. For multiplayer, everyone starts with a level 1 hero and levels up from there. The problem is that, especially on the competitive end, you run into the problems of RPGs, specifically that sometimes they can become a terrible grind. In a normal RPG, you grind and grind but then it's done and you don't have to go back to level 1 unless you volunteer to. Competitively, you have to grind repeatedly, every game, and that gets old.
Another "Strategy/RPG" title that put the two together successfully was Disgaea. The trick here was to completely eliminate any real competition between the two. It's more RPG than strategy. The "strategy" part of this game is just lifting a basic combat interface from successful strategy titles and putting it into an RPG game -- because you never play against another person, you don't mind that you win or lose based on a combination of how long and how well you play it. I'm not saying it's bad -- it's brilliant to take something from one genre and fit it into another. But ultimately, Disgaea is more an RPG than a strategy title, and has very few elements of strategy in it, and I want it recognized for what it is.
I'm not saying that crossing the streams is always bad. Hellgate: London had a lot of potential (which I don't think it reached), just using the FPS interface for an RPG. There are other titles that worked well that I don't remember off the top of my head. I just want a little more thought put into the decision making before blindly churning out another "Thing A + Thing B" game. I'm tired of "strategy/RPG" and everything else crossed with RPG. As a general rule, it seems like the RPG elements either dominate the game, or drag it down, with no happy medium.
I feel sure I'm missing some other mix-and-match that doesn't ever really work out for fundamental reasons.