The big thing about Brawl's crappy online play is, fighting game netcode needs to be perfect, or online play is going to suffer immensely for the most common use cases*.
Look at Street Fighter IV, which is "playable" online*, but is generally much laggier than Super Street Fighter II Turbo HD Remix and other games that use GGPO, even on "good" connections.
Look at The King of Fighters XIII, which is more or less unplayable online* due to SNK Playmore's decision to attempt in-house netcode on a relatively low budget, instead of spending that money on something sensible like a GGPO license.
Look at Marvel vs. Capcom 3, where instead of taking the extra time to design better netcode or spending the money to license GGPO, the devs included a lag simulation option in the game's training mode, effectively admitting that their game sucks online*.
Look at DarkStalkers Resurrection, where the guys at Iron Galaxy not only used GGPO, but optimized its use to a much greater degree than most devs who license the library; as a result, Vampire Hunter and Vampire Savior have God-tier netplay.
Platformers are in very much the same boat, but unlike fighting games, we don't exactly have good platformer-friendly netcode libraries, which is why so many platformers, both eastern and western, simply don't offer online play (or heavily downplay its existence) even when they do have multiplayer modes, rather than offering online play with half-assed netcode a la Brawl, or delaying until the netcode isn't butt.
A lot of popular competitive genres, including first-person shooters, racing games, MOBAs, and real-time strategy games, tend to have far more room for latency than fighting games or platformers; as an example, StarCraft is playable online on far worse connections (including satellite) than something like BlazBlue can handle. (There's also more room to cut corners in lieu of proper solutions to real network problems; the existence of lag-switching as a Thing That Actually Works is a good example of what said corner-cutting can lead to in practice.)
Also, "what if a player drops out and there are now fewer players than the level design was intended for?" is a wholly legitimate concern for a lot of cooperative multiplayer games. Imagine trying to solo the co-op tower in Catherine or the co-op campaign in BattleBlock Theater. You can't just have an AI take over for puzzle-solving or hidden item collection like you can for shooting in, say, Left 4 Dead, so having the computer take over for the dropped player isn't a good option, and "boot everyone out of the game if someone drops" is not fun.
*Outside of Japan. It's a lot easier to design netcode for a tiny island nation with good Internet than for somewhere huge with generally-crappy Internet, so that's what these companies do; as a result, netplay in these games tends to be completely viable in Japan, and less than good everywhere else.
