As many have already noted, the RPG genre has a somewhat shaky definition.
For many (though fortunately I'm not seeing too much of it in this topic), it still refers to D&D-derived mechanics; a.k.a.: "roll-playing games". That particular definition is becoming increasingly dated (along with being very inaccurate to begin with), as many non-RPG genres are incorporating those elements into their games.
However, the two defining elements of the RPG genre have always been the following:
- Character Customization
- Better Storytelling (and generally longer than most other games)
Character customization can easily be accomplished using D&D mechanics, though it has only been in recent years that developpers have started to go outside that paradigm for the mechanics. The Mass Effect series is a good example of that, allowing for a good deal of customization in Shepard's abilities (each class plays quite differently, particularly in ME2). The series isn't an RPG in the traditional sense, as the games use more action-oriented mechanics to fit the sci-fi setting (as opposed to the usual fantasy setting).
Better storytelling, relative to the rest of the gaming industry, is something RPGs are starting to struggle with maintaining their dominance. Many other games are working out how to tell better stories as the related technology improves and allows for more events to occur. Nevertheless, the Mass Effect games (ME2 included) still has some of the best storytelling in the business. While a bit short by older RPG standards (ME1 is 15-20 hours long, ME2 has 25-30 hours), they're still a fair bit longer than most other games these days (which run between 7-10 hours in most cases). Another very well-integrated aspect for the series is the fact the storyline itself is VERY customizable, right down to the ability to import all of your decisions between the games. ME2 doesn't exactly have massive differences between the different possible outcomes, but sometimes the way NPCs will react to you can change the mood of the game entirely; ME1 arguably did a better job of this sort of storyline customization by having more possible approaches to situations and more outcomes, but it doesn't put too much of a damper on the overall quality of ME2.
All in all, ME2 is a great RPG... unless you're looking at it from a more traditional (and dated) perspective. Even if you say it's a bit weak as an RPG, it's still an outstanding game.
