I think I have to agree with the others. The controls and level design are just so damned good it is impossible to resist.

yup what the others have said. Mario Galaxy 1 and 2 are two of the freshest experiences i have had in ages. those games are just amazingly well done and it does not matter how much Mario i get i always want more!

Great controls, level design and gameplay are definitely a plus, but I do not think it is the only reason because that doesn't help everyone else.

One can not discount the fact that Mario is pretty much a household name. There is a good chance that even people who do not play video games know who "Super Mario" is.

I think it would be hard to argue that Mario's name attributes to the quality of his games.

That isn't what I am arguing. I am just answering the question in the OP which is: "My question is, what do you feel is the most important factor regarding why Mario games tend to sell very well?"

I am asnwering it by saying it is a multitude of factors and not just the gameplay itself.

Thread makes it weird since his question and the question in the title are different.

Right you are, then.

The better question is why isn't there something better than Mario?

You pretty much nailed it although I think that Sunshine certainly polluted the franchise ironically.

Perfect controls and ingenious yet polished level design.

I do think that the Nintendo and Mario brands help sell the games.  But the fact remains that they are still the highest quality platformers out there, and consistently so.  No one else lives up to that standard time and time again, so that is why it can't help everyone else.