1) People often talk of a game made before Smash that clearly influenced the games playstyle heavily. What is the name of this game?

2) Why are there no serious imitators of Smash Bros, when games like Halo and Pokemon spawn legions of copycats?  

2) Well there was a similar game released for GCN/PS2 called DreamMix TV: World Fighters that was a mascot brawler crossover between Konami, Hudson and Takara with the likes of Optimus Prime beating on Snake while Simon Belmont took on Bomberman and more. Japan only though.

1) Kart Fighter, an NES ROM. It was a fan-made Street Fighter 2 clone using Super Mario Kart characters. People say that influenced Nintendo into putting their characters into a fighting game.

The designers for Smash say the idea of knocking your opponent out of the ring was based on Sumo Wrestling.

2) Viewtiful Joe Red Hot Rumble was pretty close to SSBM, I believe. Didn't sell too well though.

I think it's because the Smash Bros formula is too unique to copy. Smash is really platformer + fighter with a ton of fan service slopped on. Few companies do platformers or fighters, and even fewer have the cast to pull this off. Capcom maybe, but they tried with Red Hot Rumble.

Halo's an FPS and Pokemon is an RPG (I'm getting get shot for those simplifications.) Vastly easier to read the underlying blueprints of each game.

 Holy-

*faints* 

There's been a lot of imitations - Digimon, Shrek, Viewtiful Joe and lots of others, but they've all sucked compared to SSB. Play one of them and you'll really appreciate how much work and balance goes into SSB to make it the magic it is.

Love to see Sony do one with Rachet, Daxter, Sly Cooper, Crash, Kratos, etc.

Wow, people are addressing me by name to come in and answer their questions! :D In that case... 

1. The closest approximation to the game's playstyle would actually be the Kirby games on the SNES, considering that the hold R to block was a key element of the games and that HAL was the maker of both. As for the concept of mixing characters from different games into a fighting game, the King of Fighters series is probably one of the first to have done this, taking characters from the 8-bit era and bringing them together into a fighting game.

From everything I've heard, the SSB series actually started without the intention of turning it into a serious powerhouse but as a kind of tribute to Nintendo franchises. It's success was completely unprecedented.

2. There have been some on the DS as well as the aforementioned, including Jump Superstars and I believe Guilty Gear Dust Strikers also had a four player platforming fighting style. In addition, Viewtiful Joe: Red Hot Rumble was an attempt at the same style but fell drastically short of achieving anywhere near SSB's quality.

Beyond that, I honestly don't know for sure, but I'm guessing the fact that Nintendo owns so many franchises helps immensely in bringing a game like SSB together. Neither Sony or MS has direct control of many franchises which would make licensing agreements for enough characters to fill a roster an utter nightmare.

All things considered, I'm surprised there hasn't been a competitor, but I think the reason MS hasn't made a SSB clone is the same reason Nintendo hasn't made a Halo clone: MS doesn't do mascot fighters and Nintendo doesn't do FPSes. 

Oh yeah, Jump Superstars. I've heard that was an actually good SSB clone.

Nice catch on KoF.

*gasp* How did I forget about Kirby Superstar! ARGH *melts into puddle of shame*

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If I remember the story correctly, Nintendo wasn't going to publish SSB64. Despite rejection, HAL kept working on it in secret. As it neared completion, Nintendo looked at HAL's project and found it good enough. So they made a limited release.

And the rest is history.

Jump Ultimate Stars (Nintendo 2nd party) is even better http://youtube.com/watch?v=4qsuvSUMChw

^^ My 2nd favorite DS game.  Great gameplay and online 4 player wifi.  Best import ever.

Perhaps they think it's too kiddy. I've read so many comments from fanboys who prefer 1v1 fighters, that Smash Bros is just a simple party game. Maybe developers don't realize how good a 4 player fighter like Smash Bros is...

Seriously...even if it's not like Smash Bros style, are there any good 4 player fighters out there?

Although people tend to complain about anime games, Naruto Clash of Ninja: Revolution is a solid 4 player fighter.  JUS is an incredible 4 player fighter, and one of the best ever made on handhelds or systems.

Oh, and Square is going to try and copy SSB's style with Dissidia as well.