it all depends... my favorite pedals are the ones from Crank Brothers, but it's not because of the float. I like'em because of the positive feel I get from engagement/release -- with no accidental "pull-outs" under hard accelerations.

That said, I prefer pedals with no float because I have a tendency to hit my heals against the chain stays. I prefer to ride with my shoes loosened (crude methode to get float) during long easy rides only to tighten them later on when the pace is picked up during the end.

fwiw, I think there's more chance of hurting your knees with pedals with a large swing-- it would be like trying to balance your foot on the end of a pin while applying 1000 watts of power.

lack of stability...?

It was with the same ACL concerns that I moved to Speedplay Frogs for a while.  Not the slickest pedal  on the road, but f it, it's my knee.  Very happy with the (enormous) amount of float.  A couple of years after that, I moved back to Zeros.  Not sure that I noticed any difference in the reduced float.

In any event, maybe I'm understanding your characterization incorrectly, but if power transfer feels like it's an issue, I'd look at the shoe (particularly the sole flex) first.

You're worried about power transfer from the zeros?

They don't seem to hold cancellara back too much . . .

Agree w/s.j.: the shoe is a much more likely culprit in power transfer than the pedals.

(And, as an aside: why not try re-habbing on basic old platform pedals?  Once I started paying attention to where I put my feet on my beater-bike, it lead to fairly drastic changes in my pedal-position on the race bike.  Until your knee is stabilized and strong, it might be best not to lock it in too much.  Even with float, if the pedal position locks you into a rotation that's not right for your knee, it's going to cause problems.)

It might be worth having a thorough bike-fit . . .

for me (for now).  The 18 deg of non centering float gets my knees lined up just right and I've never stepped out of them in a sprint or hard effort (I have had trouble getting fully engaged after nature breaks), but once in their fine.  FWIW, I use crank bros eggbeaters on my 'cross bike (which is my winter roadie) and Candy's on my MTB.  I'm growing to really like them as well, especially the easy spd style cleat and how easily they shed dirt and snow.  They also have a nice amount of float since the design is allowed to flex a little sideways, but tighten when you pull up on the pedal.  It's a centering float, so cleat position is important, but  the cleats are a breeze to adust unlike the 2 step-screws-that-always-strip process of the speedplays.  I could very easily see myself rotating out my Speedplay fleet for Eggbeaters in the next few years.  

mixed reviews regarding float.  Some knee specialists think it's bad (something to do with the putting too much lateral strain on the tendons to keep in the right position), but others think it helps (allowing a natural motion of the knee during the pedal stroke).  I'm in the latter camp on this one.  Personally I like the float of speedplays, but maybe more than the float is the fact that the release requires no jerking motion that I found necessary with Look style pedals in my pre-speedplay days.  Once you rotate your heel outward enough degrees, you just step out.
Regarding surface area, it's a non-factor if the soles are stiff enough.  
Rode my Eggbeaters this morning on the road, and I would say the release is similar to the Speedplay's.  Just buttery smooth.  I did notice the small surface area under my feet, but I was using a pair of soft soled MTB shoes, so I was expecting it.  

Do any other pedals allow for all of the following:  Fore-aft cleat adjustment, side-to-side cleat adjustment, and float adjustment?  I don't think so.  For someone with knee problems it is probably critical to have control over all three variables.

As for float vs. fixed argument, the Zeros can be adjusted in such a way to completely eliminate all float, if need be.  I'm no doctor, but common sense tells me that you should probably have a pedal system that would give you options either way.

Lastly, if you feel power transfer is an issue, I'd try a new shoe with a stiffer sole before switching pedal systems.

FWIW, I've tried a variety of fixed clipless systems over the past twenty years. I switched to the Zeros about six years ago in an attempt to resolve knee pain, which eventually went away by dialing in the cleat position.

Good luck on the rehab.

I'm building out a new bike and thinking about switching pedals so this is great info for me. Thanks for the knowledge...

But it's going to be a while before I actually take delivery. They are pretty busy I gather. But no worries... the wait just gives me more time to mull over options for the rest of the bike.

I wish I could afford Record but at this point it's really a question of coming in under a total budget that will not cause my wife to divorce me. Probably going to be Ultegra to start with. I would love to get Record or SRAM but my bag of money is a little light after getting the frame and I can always upgrade stuff later. I will go as light as I can afford on the wheels. Speaking of which, any suggestions on affordable and light wheels? Are those Neuvation wheels any good? Someone was talking about them here at the cafe a while back...