I have and am running a board for freeform supers games. Here's my opinions and observations, for what they are worth.
I disagree that freeform attracts a certain kind of player. I've had munchkns and thespians in my freeforms. But I've also had them in my system oriented games. The only type I have noticed that doesn't ever seem to RTJ are the rules lawyers. Or perhaps they do apply but chuck their expectations once they see it's freeform. I don't know.
I do agree wholeheartedly that you need to set expectations from the get go. This is more important than in system oriented games. In a system game (D&D, Paranoia, etc.) the system sets up the expectation. You don't get that in a freeform game. Just saying it's a supers game isn't enough. Is it X-Men or is it Watchmen? Two very different styles of gaming. Is it Spider Man or is it Superman? Again, very different. You need to make it clear from the get go where you expect the game to fall. Four color. Iron Age. Slice of life. What have you.
As far as power levels, if you're running a group, yeah they probably need to be consistent. But if it's small group or solo, let 'em play Superman or Silver Surfer. Again, make sure you've communicated things up front.
I stopped running large group super freeforms. They don't work in my opinion. Honestly, I don't think large groups of any kind work on PbP. But freeform can especially get messy the more players there are, mostly for the reasons already stated in earlier posts. I run solo sandboxes. At most I would run a freeform game with three people in it.
Freeform lends itself well to making things up on the fly or improving an adventure. If you don't feel comfortable riffing an adventure on the fly, you may want to rethink running freeform. Might just be me, but I have found it much more difficult to keep a story on a preset flight plan in freeform. Players tend to get a lot more creative when they don't have to worry about initiative or saving throws. Expect all your plans to go south even faster than they would in a system game.
I use different methods to handle combat in my games. It varies depending on the player. But approaches I have tried:
-Pure Narration. Either I or the player will narrate what happens.
Pros: With the right player, this can work fantastically. You'll know
you have a diamond when the player narrates their own character
stumbling or taking a hit.
Cons: The operative part of the Pro was "with the right player". If the
player seems to be a munchkin, letting them go nuts is not
advisable. They of course will destroy everyone with one mighty
blow. All the time. Every time. You get bored. They get bored.
-Single die roll. Have the player roll a die. The higher the number, the
better the result.
Pros: Quick and easy.
Cons: Completely random. Player's input into test results is limited.
-Moving Target in a Range. Set a scale, say 1 to 10. Within that scale, pick the
target number you think reflects the difficulty. Have the player pick a number,
either at random or by rolling a die. Then let the player modify that result
within a small range. The optimal result is that they tie the target number.
The father away they are from the target number, either over or under, the less
successful they are.
Ex. Bouncing Betty is trying to bounce off a wall and hit Underwear Man from behind.
Bouncing Betty bounces. It's in the name. Underwear Man is busy snickering at a bus
full of burning orphans. On a scale of 1 to 10, 10 being hardest, you set the target
at 4. Feels right to you at the time. You tell Betty she has to roll a d10 and can
adjust the number by 2, either upwards or downwards. She rolls a 7. She thinks
it will be easy and adjust down by 2. Final result 5. Pretty close but not perfect
You narrate the results.
Pros: Crunchy enough to satisfy most players. Not something you have to
think a lot about ahead of time/easy to work on the fly. Player
gets some input through the adjustment of their die result.
Cons: Can get confusing at times. Works best when only used occasionally.
Otherwise, why not just use a system?
On a side note, not all my super games necessarily involve combat. It can be done and with the right player can work beautifully. But again..."with the right player".
Anyway. That's probably more than you wanted to hear. Sorry for the long post. Best of luck with the game.