We've had this topic before in a more specific form, but here I'd like to expand upon it. I know of a few people who are die-hard purists, professing such love for a given genre that they will either refuse to play anything other than their favorite, or play every example of that genre regardless of quality. The opposite is also true: players who hate, say, turn-based strategy games and proclaim that only the RTS is worth playing. Being vociferously exclusive is fun, sure, but it's difficult to make the case that any one genre is inherently superior to any other. It's really all in how the genre is applied.

There is also a rather large simmering debate between the primacy of science fiction over fantasy, and vice versa. Again, application is everything. More so, in fact, because it's a relatively simple matter to retool the settings, names, and plots of one to fit in with the conventions of the other. They're just window dressings to spice up whatever theme lies beneath the main story.

Now, the issue gets even more complicated when you combine functional genres with thematic genres. For instance, a first-person science fiction shooter (ex.: Deus Ex, or Halo), or a fantasy-themed MMO (World of Warcraft, or Warhammer Online). There have been fantasy FPSes before (Hexen, Bioshock), and science fiction-themed MMOs (EVE Online, Tabula Rasa) as well. When judging the success of either, hanging it on the genre itself is an indefensible position. There do, however, seem to be fewer science fiction MMO success stories than the converse, and fewer fantasy FPSes as well. Is this an example of the better genre for a gametype pulling ahead of the competition, or simply watching developers try to ape the popularity of a particular best-selling title?

There are gametypes that seem to be self-limiting. MMOs in particular require an environment with enough variability to offer a wide range of choices -- not only of character class, but also powers, set pieces, and scenarios. Fantasy and science fiction both provide Macguffins aplenty for nearly any type of situation you'd like to invent. Thematic genres like the Western or the Mystery are much more locked-down. Obviously I'm not saying it's impossible, but it's certainly more difficult to craft something compelling from a genre with rigid boundaries.

At the same time, there is something to be said for deliberately flaunting convention.