I find Smash one of the most balanced fighting games out there... I'm sure many would agree.
Sure the Tiers are apparent, You'll always have tons of Shiek/Fox/Marth/Peaches at Tournaments, but I find it has alot more to do with Mind-games and Skill than most other fighting games. a Lower Tier'd character can still beat a higher tier character with enough skill/mind-games/etc...
You look at other games like...
Marvel vs Capcom 2; its a Magneto/Storm/Psyloke/Sentinel/Cable orgy at tournaments.
Marvel vs Capcom 1; Wolverine/Strider Hiryu with Colossus Helper, every time.
Capcom vs SNK 2; Blanka/Bison/Sagat/Sakura every which way
Capcom Fighting Evolution; Don't get me started on the beasting of Jedah and Anakaris
Street Fighter 3rd Strike; Someone say Ken?
Of course occasionally with these games you'll have those people who come with these weird mid-tier teams (MikeZ's Juggernaut/Tron Bonne/Dr.Doom) and still come out on top because of their high skill, but usually the above mentioned characters play on just a completely different level of gameplay than the lower tier characters. I personally find this bar to be much more blurred out when it comes to Smash... Team Darkness being a good example of that ;P
I also find that in Smash, alot of people choose a character that fits best with their playstyle, and that they like the most (for a variety of reasons, aesthetics, nostalgia, representation, etc...) whereas in other games they pick characters based off their position in the tier-list. I know for myself when the game was first released, I took Ice Climbers and stuck with them since I just found the idea so cool to have the 2 IC's following eachother. Even though they aren't placed very high on the tier list, I still stick with them. Whereas in a game like CvS2, I remember starting off with like Ryu/Joe/Mai, and now I use Sakura/Hibiki/Rock, because its just so hard to win with low tier characters so I had to change to a higher tier if I stood a chance against my friends.