100cc proceeds the 50cc engine class, picking up both added speed and difficulty. The game states it is for average players, because it's not too hard or too easy. Primarily, only Bikes can be driven with 100cc. Like the other engine classes at the start, only the 4 cups are open to race on, 2 apiece for the Nitro and Retro Grand Prixes. The other must be unlocked by beating the default cups.
The racers actually go X2 faster than 50cc, which can throw off some people. Corners are now harder to cut, bumping into people causes a harder knockback (but it's not a problem for Large Characters, though), and laps can now be made in less time.
The hardness of the CPUs is raised in 100cc; they now go just as fast as the human player/s, making for some pretty close races. Items thrown/used are still moderate though, there's no extra Blue Shells or anything like that. Mario Kart can be unfairly cruel, but not that cruel...
Bikes are the only accessible vehicle to use in 100cc (at first). Wheelies can now be performed too thanks to 100cc's Bike allowance, so it's just more and more speed. The only downside to Bikes is that they have less Handling capabilities compared to Karts, so be prepared to run into walls... As hinted at earlier in this paragraph, you can switch the vehicle acceptance on the Engine Class Selection Screen after all cups in 100cc have been beaten with top rank. Just hit the + button, and both types will be allowed.