What are they?

I like the Mario Adventure games... but have no interest in Mario Kart or SuperSmashBros since Ive already been playing those games for the past 10 years.

I know there is MadWorld...Conduit and PunchOut coming soon, and I am looking forward to both (mainly PunchOut)... anything else???

Resident evil 4 is amazingly good, but hey you can have fun with most wii games out there despite your age.

I really recommend mario kart despite having played it the past 10 years.. but if you dont take my recommendation Id suggest getting Little king's story.

american idol

I think this is a key point.  Nintendo is the master of games that bely their appearance.  That is too say deep mechanics under a colourful exterior.

Mario Kart online is awesome and challenging if you are up for it, there are some amazingly good players out there.  I'm sure you are aware of it but i have to recommend Metroid Prime: Corruption as a truly unique and awesome experience.

Over 20 years old doesn't narrow it down enough. I'm 26, and as I've been growing out of the teen video game demographic, my tastes have been getting narrower in some areas while expanding in other areas.

Is there any genre you're in the mood for?

Damnit, I'm turning 20 this year. Does this mean that I have to stop playing all of these Wii games I own, since I'll be 20 and my tastes are strictly changing?

No More Heroes is a must buy, especially the bloody american version.
After that, I'd go with The House of the Dead:Overkill, Mad World and Manhunt 2.

If you want some "not so bloody" games, then I'd go with Zelda, Galaxy, Excitebots, Little King's Story, Fire Emblem, Metroid.
These are the first which come up in my mind.

Mario Galaxy?

Im 24.

Mario Galaxy
Resident Evil 4
Resident Evil Umbrella Chronicles
Resident Evil The Darkside Chronicles
Dead Space Extraction
The House of the Dead II & III
The House of the Dead Overkill
Muramasa: The Demon Balde
Arc Rise Fantasia
Deadly Creatures
Coursed Mountain
Fatal Frame IV
Feel
Silent Hill (The Remake)
No More Heroes
No More Heroes 2
The Conduit
MadWorld
Punch-Out
Dead Riding: Chop Till you Drop

if you like puzzle games and can look past a colorful exterior, then definitely zack and wiki.

super paper mario is a good platformer with some interesting gameplay.