Burned out on Diablo 3, waiting for World of Warcraft's next expansion, I've crawled back to my Xbox 360. While the selection of classics isn't as good as Wii's there are some addictive games to be found in the arcade section, and not all of them cost fifteen bucks.
In no particular order, here's some of my favorites:
Plants vs Zombies - As a strategy game, the title tells you all you need to know. You raise plants that shoot peas, turn to bombs, catapults, etc. and defend your house from an invasion of zombies. The Xbox Live version comes with multiplayer, gardens, survival mode, bowling, puzzles, and purchaseable/unlockable items.
Shadow Complex - This platform-adventure reminds me a lot of the original Metroid. As a 2.5D side-scroller, you crawl through vents, take out armed henchmen, and find weapons/upgrades/ammo. You even level. There's also a challenge mode, but I never got into it. Just like Metroid, you might not be able to access areas until you obtain a specific ability or weapon, which means you backtrack and search.
Dungeon Defenders or Orcs Must Die - Dungeon Defenders is a tower defense game with multiple classes (none of them play alike), a leveling aspect, endless builds, and gear drops like an old-fashioned action rpg. My only complaint about this other than there not being enough flexibility with the camera angles is that it was released on consoles prior to the PC's release of 8 additional classes (countess, adept, ranger, initiate, summoner, barbarian, series ev, jester), all of which are amazing. Because of Xbox's limitations for downloadable content (DLC) Xbox users are stuck with only 4 classes and will never get to experience the game in full. If you didn't know this, however, the game is a blast.
I would also recommend a tower defense game called Orcs Must Die. The mechanics are faster and smoother than Dungeon Defenders, so is the camera. Though it only has a single class/character with 4 different, somewhat strict builds after you make the initial choice, you build him with each level and change depending on the set-up. Lots of carnage, interactive maps, it's also half the price of Dungeon Defenders.
Castle Crashers - A 4-player hack and slash side-scroller with rpg elements. You pick from a wide assortment of knights, kings, and villains (assuming you grab the cheap DLCs) and travel to different levels/zones smashing minions. On the way, you can find weapons and pets, each with distinct abilities. Beating the game offers a +mode with more of a challenge. Unfortunately, there was never a legit expansion, only DLC with new character choices to repeat what you've already done (really not necessary to level more than one character), and The Behemoth company doesn't make sequels.
TMNT: Turtles in Time Reshelled - A revamped classic and model as to what I want for my old games--updated graphics and everything else the same. I immediately thought of my childhood and all the fun I had playing, mashing buttons, kicking foot soldiers at the screen, yelping, "My toe! my toe!" and stomping the Super Shredder...good times. A bunch of other classics I picked up didn't grab me the way the originals did, such as 1942: Joint Strike or Space Invaders Extreme. TMNT was as great as I remembered and more.