I grew up on the 16 bit and 3D eras, so going back to play 8-Bit games can sometimes be a chore for me. Unless the game is absolutely great, I tend to lose interest rather quickly. It's hard to have motivation to complete old games when you don't have any nostalgia to help justify the reason you're playing it in the first place. I didn't experience this problem with Kirby's Adventure, the game could have easily been released on the Super Nintendo with nothing but a graphical update, and maybe even without one.
The graphical presentation is mind melting for a NES game. Granted, it was released in 1993, but this and Metal Storm are definitely the pinnacle of what the NES was capable of. The ending sequence is especially beautiful, and would even hold up today alongside other 2D titles. The game is vibrant in its colors, and some of the things employed, such as a "3D" affect on a spinning tower, are almost mind blowing given the console.
Level design is as equally impressive as the graphics. One level in particular, the last one before the final boss fight, will likely stay in my video game knowledge bank forever. Each new screen is a piece of a level you've already visited, one from each world. A remixed version of the first level's song plays as you make your way through it, and it's a great tribute to the rest of the game.
I'm sure most of you are aware of Kirby's ability. He eats other enemies and gains powers from them from doing so, such as turning into a fireball or singing into a microphone so loud everything on the screen dies. He can slide, eat, fly, and jump. Your main attack is sucking up the enemies, and you can also shoot them at other enemies should they not hold a powerup. Other attacks, such as Sword Kirby and Fireball Kirby are gained by eating enemies in the fashion I explained above. It all comes together to create a great gaming experience.
The last thing I want to touch on is the games ending. I won't spoil it for you, but the twist at the very end is was unexpected, and the boss fight was one of the best I've ever played. In short, if you own a Wii you should download this game. It's only 500 Wii Points and you can always buy Mega Man 2 with the remainder. You could also look for the GBA remake of it.