For me, the one and only thing that separates Hawken from almost all the other FPS games I have played is that friendly fire is turned off. And it doesn't even make sense why.
You can do self-damage. How in the heck does it make sense that your weapons can damage your own mech, but your team is magically invulnerable? In addition, you can use items on your teammates (EMP, Repair Charge, etc.) but for SOME reason you can not damage your teammates. It is a never-ending battle for this game, and most other games as well, for people to continuously claim "This of OP", or "This is IMBA". Well, when you add friendly fire into the mix, it solves a lot of the root issues that such complaints stem from
Number one, adding friendly fire would most definitely ruin certain mech types, especially light mechs. But it would also force these types of mechs to not just rush in forward, with hopes that their team will be right behind them firing away. In the case of friendly fire, this same situation would end up with the light mech getting annihilated. Also, it would allow stealth mechs a much bigger advantage, because the enemy team could not just fire into their own group when a Predator or Infiltrator gets caught too close behind enemy lines. Another type of mech that would be greatly affected is the Rocketeer and Bruiser. Hellfire missiles would not be abused as much, because they would easily hit their own team mates if not targeted correctly. Snipers would be forced to time their shots to avoid team mates when protecting them from a distance.
Number two, I think most people can agree that TDM's are won and lost based on how well either team works together and friendly fire would only add to that. It would force people to think twice about where and when they shoot. It would also slow the game down a bit. Yeah, some new players might not be helped by this change, and they might cause a lot of unintentional deaths. But the fact remains, it is extremely embarrassing to kill your own teammates and that fact alone would prevent these situations from happening as commonly as you might expect.
Please feel free to add your own opinions!