Why am I getting involved in this conversation...
I'd like to make a point that it isn't the amount of thinking you have to do that makes a game competitive. If this were true, we'd all be playing reversi and tetris.
http://www.youtube.com/watch?v=59y2KLDT_1Y
While I know that Halo 3 wasn't the greatest game ever to be released, it falls on the same concept of why Brawl is popular, because it's a sequel to a really really good game. Nevertheless even with a dying fanbase and newer, shinier, more bloody, testosterone filled games each with their completely original concept of mass genocide are coming out, it doesn't stop halo from being able to stand on its own feet as its own game. I have an inkling that if Brawl was released as it's own game and Melee and 64 never existed, there would be a substantially less amount of people playing it, and it would be selling even less copies in compairison to Wii Fit.
While I personally love Halo 3, moreso than its prequel for personal reasons, I have no idea why people had so eagerly left it for Halo 2, while others have stuck with games like Melee, ST, and GGXX, even though their shiny new adversaries have come out.
We can all agree that depth isn't what's making the MLG curcuit revolve anymore, or at least not as much as it was, and games are suffering from pretty bad cases of sequelitis.