Forgive me for just copying my answer from another post:

"What can be said about Super Mario? The first game in the series is probably the most important piece of video game software ever made, a title that resurrected an industry on the brink of collapse and made Japan the epicenter of the video game world. Almost thirty years later, the series is still the best platform franchise around. What's most remarkable about Super Mario is that it dominates both 2D and 3D platform games: Super Mario 64 and Super Mario Galaxy are two of the best and most groundbreaking 3D platform games; Super Mario Bros. 3, Super Mario World, and, more recently, New Super Mario Bros., are some of the very best 2D platform games. Even when the series strays from the formula -- e.g., Super Mario Sunshine, Super Mario Land -- the results are outstanding."

As far as Metroid goes, it deserves more attention than it gets. I would rank it among the top three franchises ever made.

Everything about Mario is perfect for a video game hero. His image is average joe, but not whimpy. His colors are blue and red, which appeal to the human brain (which is why so many flags/sports teams use them). He has a few distinctive characterstics so we recognise him, but for the most part we know little about who he is so he can be anything we want when we play a game.

Mario's games have been nothing short of the gold standard of the industry. For anyone saying Super Mario Bros wasn't that great... wow, I feel like you didn't live through 1985. SMB was groundbreaking... Mario didn't become huge because of being included with the NES, the NES became huge because of Super Mario Bros. Everyone HAD to play that massive game full of secrets and levels and hidden power ups. And unlike so many other games, it was both challenging and accessible, free of guns or cheezy sci-fi themes that might offend some. The play control was second to none.

And Mario had already established himself with the exceptional Donkey Kong a few years earlier. Since that time games like Mario 3, Mario 64 and Mario Galaxy have continued to be undisputed masterpieces, pushing technical and gameplay envelopes while maintaining simple basic mechanics and themes that allow almost anyone to pick up and play.

This long string of success and unwaivering quality, an excellent basic theme and great marketing by Nintendo have mad Mario a exceptional brand icon. Hiroshi Yamauchi knew exactly what he was doing with Mario, as his inspiration was Disney and its character branding who he originally licensed before creating his own. At this stage, on top of everything else, we also have a powerful nostalgia factor, with many of us having our first great gaming experiences with Mario.

Metroid is no where close. I really enjoyed Metroid and Super Metroid as a kid. But those games are weird. A lot of people don't like weird alien settings. They don't like a faceless robot hero. They don't like to think too much and back-track or shooting. Metroid games are excellent if you like that style, but they have nowhere near the universal appeal of Mario games, and the theme and characters are no where near as appealing to masses, nor have they been promoted in the same way.

Because everything in it is designed to be intuitive (you see a goomba, you want to jump on it, etc)

It has no big scenario, so it's left to gamer imagination.

And a lot of other stuff like that that makes it an instant classic for all ages, from casual to hardcore.

I mean, I started playing Super Mario World when I was 2. And I'm sure I'm not the only one.

Found this...

I think this is the most impressive spambot post I have ever seen.

It is because of the Super Mario Bro's. Movie, obviously we need a Metroid movie, big budget ordeal.