Plot
Kirby's Adventure's plot is a simple one, but a sweet one, and it helped mold many later aspects in the series. Basically, the Star Rod has been stolen, and King Dedede (the infamous penguin villain in the last game) was found bathing in the waters of the Fountain of Dreams, a spring where dreams come true. It's up to Kirby to find the pieces of the wand and fight Nightmare. Not only was this games' plot set up well, it actually had an ending worth watching---no doubt one of the best in the series.
Gameplay
Solid gameplay reminiscent of Super Mario Bros. that's easy to pick up and play, whether you're a newer or longtime gamer. The addition of copy abilities that allow the puffy hero to gain new powers by inhaling different enemies is genius, and since then has been copied endlessly in every Kirby game (including the anime). There are also Hidden Switches and a 100% file completion feature, which drastically add to the replayability. The gameplay isn't the best ever, but it's definitely hard to ignore the feats it accomplished.
Graphics
Easily the best seen on the original Nintendo. While the game has an advantage, being released two years after the system's death, you have to admit they are quite nice. The colors match the cartoony, vibrant settings just right, and it's quite a step up from the black-and-white adventure of Kirby's Dream Land. They're also some of the most recognizable and timeless of all the Kirby series. They might seem simple at first glance, but the framerate of sprites and surprising amount of detail is flat-out incredible when compared to earlier NES titles of The Legend of Zelda, Kid Icarus and the like. Truly spectacular.
Sounds
Everything sounds nicely arranged, with some really memorable tunes in there that have become somewhat of a legend to the Kirby series. A lot of tracks have been remixed and reused in different games, such as the awesome Butter Building theme in the recently released Super Smash Bros. Brawl. The steady congo beat in the background can be mildly distracting, but adds a new flavor to the music. Sound effects are all nice as usual.
Contribution to the Series
Kirby's Adventure is a true classic, and another one of my favorites. It's fun to play and really built up the weak spots of the original Kirby's Dream Land.
Plot: 9/10
Gameplay: 9/10
Graphics: 10/10
Sounds: 9/10
Contribution: 10/10
Overall: 8.7/10