Exploration. It's called exploration for those who want it. And most will pass right by a good amount of it, while the more dedicated have something to look forward too.
Okay, it's not a popular opinion, but I'm gonna say it anyway. 2-D Sonic is better than 2-D Mario. Always has been (except the Sonic 4 series and a few other exceptions).
Now, don't get me wrong, when it comes to 3-D platformers, Mario blows Sonic out of the water. Even Sonic's best 3-D game (Generations) doesn't stand up to Mario 64 or Odessy, but when it comes to 2-D platformers, Mario has become stale.
The original Super Mario brothers up to Super Mario 3 are great! For their time. But the fact is, Nintendo really hadn't done anything with the formula to make it fresh over the years. However, the New Super Mario Bros series is flat out BORING! Hell, I never finished Super Mario World.
The speed, physics, and exploration of the Sonic series has always felt fresh in my eyes. It went from rolling to occasional slow platforming in Sonic 1, the spindash and Tails in Sonic 2, and shields in Sonic 3. All this with incredible bonus stages to add variety to the mix and the most incredible soundtrack. As you can tell by the top image, speed isn't the only thing to do in a Sonic game. You can explore, find shortcuts and bonuses. Find the best path to speed run. What can you do in a 2-D Mario game? Go from left to right. Pick up some power-ups along the way, go down a pipe for a simple bonus stage with the same gameplay, warp back, then jump down a flag pole. The same shit he's been doing for 35 years! Sonic isn't afraid to change up the formula a bit, even if it sometimes leads to some bad 2-D games. But then we get games like Sonic Rush where I'm boosting at high speeds while music from Hideki Naganmua (Jet Set Radio) is playing. Meanwhile, in NSMB, you get the same music theme over and over again. "Bop! Bop! Ba-dop!" NSMB is so BORING!! The worst series in the Mario franchise IMO. Although I did like the gold coin collecting in the second one.