Well, I'd have to explain afew things first. In fighting games, I think it's fun to get good with a character, learn his/her moves and sort of form a link with them. I don't like doing the same moves over and over, and I don't like cheap wins, like kicking someone in the foot to deplete the last of their life meter. In SSBM, I get to be Mario, which is great for me, since I already have that closeness with him. As you get better in the game, you can try harder things. You can try and get the trophies as you go along (I have 297 of them!), and you can work on the event modes. You can master Classic and Adventure modes, you can do Tournaments, and all the Special Modes. Once you beat enough stuff, you get to hear the wonderfully orchestrated music. I like doing team matches with my brother (Adam) as Mario and Luigi, I like big tournaments, things like that. Every fight presents a new challenge and every fight is fun.
Trying out different stages as you get better is fun, too. I'd say you have to have good maneuverability to fight well on the Big Blue stage, and you'd have to have patience and experience on the Rainbow Cruise stage. There are classed enemies and new ones introduced, the same with modes and stages. It never runs out of fun as you can change the items, hit ratio, etc. There are a million possiblities for fighting, and that just in "Melee".
Once you get good enough to you get down to the nitty gritty, like being one to carefully select your costume, or dodging a lot as a fast character, or becoming an expert with Mario's cape whip (I'm close!).
I couldn't say enough on this game. But, it is a fighting game, so naturally it lacks of the drama of games with more storyline. It's a connection from Luigi's Mansion to Super Mario Sunshine, in a way, and it's also fun enough that it isn't just "something to do".
And that's what I think. Ciao.
See how the mustache hides the slightly sad smile... and the sideburns, portraying his simple, blue-collar roots... and his legs, thick, wide, the legs of the masses!