So I got a chance to get some hands on time with the game, and I've got to say, I had a bit of fun with Smash Run (even if I just kept coming up in the same spot). It looked like there was a cap on loot acquired during a single Smash Run, and Speed and Jump stats cap out at 1000 (I ended up getting an event that turned my Samus into an orange blur). I ended up using Samus, Zero Suit Samus, Greninja, Megaman, Marth, Kirby, Wii Fit Trainer, Donkey Kong, and Zelda during this run and here are my thoughts.
Samus did indeed get a noticeable buff finally, Zero Suit was fun but I had a hard time closing with her, Greninja was just sick nasty in the air or with his Side-B, I don't know if it was meant to be a humorous reference to MM2 or just the way things ended up but Megaman's P-Shooter shot is Forward-A with Metal Blade being Neutral-A, Marth felt a little less OP'd than he did in Brawl but otherwise he's the same, I couldn't get Kirby to work for me, Wii Fit was excellent for haymaking in general, Donkey Kong seemed to have a slightly better air game than before but I kinda got worked over before I could really sink my teeth into him, and same for Zelda though her point attacks seemed to be easier to land but her Din's Fire and Nayru's Love got debuffed slightly. I was able to land Final Smashes with Greninja and Kirby (though it took a lot of set up).