Addictive, Accessible and Remarkably Memorable
The most notable piece of the game is simply the addictiveness of the controls and gameplay. The reason this franchise has existed and survived so well over the course of 4 games is that the fans want it that way. Controls are accessible, the characters are fun and playing the game a second time through on a high difficulty with the full roster of weapons is a blast.
It's even the little things...using the Swingshot, gravity boots, etc. This game offers a tremendous amount of variety in combat. There's something like 14 different plants/levels to visit.
In fact, my only complaint is to wish that some of the individual levels were a bit longer - many of them were quick fetch missions. Fewer but longer levels may have done a bit more for this, but that's really the only thing.
Reviewing the graphics is difficult due to the game's age, but it looks sharp, colorful and works great. The music's catchy. All is well and good.