Mr. In-between said:
I usually play Nintendo games like Zelda, Mario, etc. But I am slowly getting into shooters and I've been into RPGs for a long time.
With this in mind allow me to reccomend a short list of games that you might like:
1- Mass Effect: This is probably one of the best RPG's on the Xbox I've ever seen so the whole series is worth checking out (also has a bit of shooter to it if that's to your liking).
2- Bioshock: Another blend of shooter and RPG but it does have something of a dark and dystopian mood to it (not sure if that's a bit of a problem considering your preference for Zelda and Mario stuff but if you don't mind darkness in games then it's also definately worth checking out).
3- Fallout 3/New Vegas: Someone else did reccomend Oblivion but in my experience...it wasn't very good, however, the Fallout games that followed it I actually did find to be suprising with a lot of the things that bugged me from Oblivion having been revised and refined and with more of a sense of personality and uniqueness than the relatively bland and generic setting and population of the Elder Scrolls series. Worth a check if you're after a mildly insane post-apocolyptic RPG.
4- Brutal Legend: A bit of a weird combination of Hack and Slash and RTS but I'd highly reccomend it for it's story, characterisation and the world alone (not to mention that once you get into it the gameplay is actually pretty good), I probably should mention that this one does get a bit of an easier pass from be because I'm a massive Jack Back fan (please don't hurl whiskey bottles at my face).
5- Perfect Dark Zero: I have to reccomend this one largely because it's very much a callback to a lot of the old shooters like Goldeneye and Timesplitters, if you liked those games and want something like them on the Xbox then these are worth a look (also, the Perfect Dark remake on XBLA is even better for this).
6- Castle Crashers: An XBLA title that is very reminiscent of Golden Axe and it's ilk except with dark cartoon humour and RPG elements, not much more I can say really. Definately good for playing with friends as well.
And I'll leave my suggestions at that before this becomes essay length, if you want anymore reccomendations or have any other questions then by all means feel free to ask (I'm happy to help).