Let me make it clear, I have no direct nostalgia for old Sonic games. I didn't have a Sega Genesis growing up and was therefore more partial to Mario and his mushroom chomping tendencies. Given that, I was honestly surprised then to find myself digging the silly little blue guy in Sonic 4.
I think what I love the most about it is the simple gameplay. How many times recently have you wanted to accomplish something in a game but you couldn't remember if that action was relegated to Right-Left Upper Bumper or pushing the Right Analog Stick three times before swinging the Left Stick Up and then Down, all while hopping on one foot? I exaggerate, and I don't even have Kinect yet, but there's something to be said for a game that is just easy to control. In Sonic 4, you jump by pressing A or B. You can perform a homing jump on anything with a red target on it while you're in the air by also pressing A or B. That's pretty much it.
Collecting rings is still what Sonic all about. If you hit an enemy or some spikes that would hurt you, you drop all of your rings. If you don't have any rings when you get hit, you lose a life and have to start back at a checkpoint. The simple ring mechanic drives you to always collect more rings as you progress through the level.
But that simplicity doesn't mean that the game feels threadbare. There is a surprising amount of complexity and difficulty that can be derived from such simple controls. And it's all about the level design by Dimps and the Sonic Team. Sure, many of the themes and concepts from the first few games are present, especially in the loops and spirals of the early levels. There were casino-themed levels in Sonic 2 and 3 but the Casino Street Zone in this one doesn't feel derivative. It's just fun bouncing Sonic around like he was pinball, hitting all of the cards to net you extra lives. The graphics utilize the foreground well so that you don't feel like you're just playing in a flat 2D world.
My favorite levels were the ones in which there is a genuine need for Sonic to go fast or he will die. Whether it's a huge machine chewing away at the robot factory, or an entire level played underwater, the looming danger pushes the action forward and keeps you on your toes. Speed has always been Sonic's strength and it's great that lead designer Hirokazu Yasuhara understood that.
One particular level deserves mentioning: the World of Darkness level in which Sonic holds a torch and can only see as far as that tiny flame illuminates. Sonic can ignite the sconces on the walls not by pressing an extra button or shooting flames, but by simply running by them. This mechanic is used throughout the whole level to unlock power-ups and extra rings, or to solve puzzles. I just thought that it was such a great and simple way to design a challenging, original level without complicating the controls or messing with the core gameplay.