8 wasn't the first Mario Kart to feature guest characters. DS had R.O.B., and the arcade spin-offs had Pac-Man, Ms. Pac-Man, Blinky, and the Taiko drum.
I forgot ROB. I pretty much consider the Arcade ones spin-offs though.
The characters sell it, but it's the gameplay that keeps people going. No one would be playing any of these games if they played like crap. Gameplay is still first and foremost, but the characters are a very important aspect.
But that's the thing. It's a Nintendo product. Very seldom do they put out a product that doesn't have at least fun gameplay. Fun gameplay is a given with a majority of their products. If anything the characters and gameplay should be equal. And Sakurai treats them as such.
Because let's be honest, there's not a damn thing wrong with the gameplay of Brawl and Wii U/3DS. Melee just has a certain set of rules and meta parts that were left behind from a man primarily overlooking them because he was doing a good portion of it on his own. Not that Melee isn't fun itself, again a Nintendo product.
Characters should equal to the gameplay and should definitely be the selling point. No one is even going to play the game to find out if the gameplay is worth it if the character roster is lacking.
Look at Mario Kart 7, lots of people hated that roster.
Look at PlayStation Allstars Battle Royale. That had a fair share of characters people enjoy, but also messed up it's gameplay and it's roster.
Both things matter. But the first thing that is going to capture attention to get someone to play a game in a crossover? ''Does it have my favorite character?"