I love how so many people are sitting here stroking their beards and saying "hmm, well, yes, the man has his opinion, we shall let him be." Because, you know, that's constructive.
Anywho, Mario and Ancel's creations are like New York Thin-Crust and Chicago Deep-Dish Pizza: both fantastic, but for very different reasons, and truth be told rather incomparable (despite both being bracketed as platformers or pizzas).
I never really found the worlds Mario inhabited terribly compelling, but the game play is always and consistently top-notch, which makes for a great experience. You can't NOT have the world of Goombas, Pipes, Koopas, etc., but, outside of brand identification, the game wouldn't be dramatically different if it were a man in space jumping on moon trees to save his co-pilot from a giant termite. In short, the visual elements of Mario's world are relegated to the status of iconic, which is fine, but places damn near 100% of the game's emphasis solely on game play.
Ancel's products, on the other hand, weave game play in with all of the other elements of the game: music, visuals, aesthetic, story, delivery, etc. It is a more "total" experience, in a sense: whereas a Mario game's strength lies in how the game plays, Rayman and BG&E rely on everything working, arguable in concert. Now, Ancel's games don't always show the same level of tightness in play mechanics a Mario game will, but you are inherently getting a totally different experience from Rayman than Mario. It's just as much about the music and the monsters in Rayman as it is the platforming.
Again, both hold very special places in my heart, just for different reasons. I can understand Ancel finding Mario unappealing, because he sees (and makes) games in a very different manner than Mario games are made.