1) They wrote off the 2008 season early on and developed the 2009 season car once the 2009 regulations first came out.
2) They got Ross Brawn, who got turned down by Ferrari to be the team principal in 2008; remember, this guy is one of the main people responsible to turn Ferrari around in the mid 90s. (imo, big mistake on Ferrari's part)
3) Innovative interpretation of the 2009 regulation on aerodynamics, they got an diffuser which creates more downforce than other teams (other than the other 2 teams who interpreted the regulations the same way)
4) They got the lowest-hanging nose allowed by the regulations (it's also very wide compared to other teams.) See pic of the nose to see how it dips down:
Many people are saying that it contributes to BrawnGP's dominance in testing...
Just imagine if they debut their cars earlier and got more testing time! I wouldn't be surprised if they dominate this season... Quite frankly, I am happy for Jenson Button.
Honda shouldn't have pulled out, just imagine the publicity they would get! Afterall, they are still paying the bills for the 2009 season... at the same time, I am glad that BrawnGP got the Mercedes which is really reliable.