well, let's see...
Does MnM have Defense instead of AC?
yes, if by Defense you mean an attribute the player controls when it increases.
just like characters in M&M have a
Base Attack Bonus, they also have a
Base Defense Bonus which is the basis for their Defense score. Dexterity, Super-Dexterity, Super-Speed, the Dodge feat, and other things can also increase Defense.
How does it handle reducing damage, say from wearing armor or being invulnerable?
there's really two different ways of reducing damage. the simple way is to simply buy up your Damage save (which is based on your Con score, just like Fortitude saves) with the Amazing Save power.
the second way is to buy some form of Protection: the Protection power itself, Force Field, or Armor. Protection directly reduces the damage level of the attack,
before you make the damage save.
for example, someone hits you with a +10 damage Energy Blast. you have Protection +8. normally, you would have to make a Damage save at DC 25 (15 + 10 for the attack's power); but with the Protection you now only need to make a Damage save at DC 17 (15 + 10 - 8 for your Protection level).
if your Protection level is strictly greater than the attack's level, it "bounces;" i.e., it does no damage whatsoever. no Damage save needed.
What applies "to hit", Str or Dex?
Str for melee, Dex for ranged, just like the rest of d20. with the Attack Finesse feat, you can use Dex for all melee attacks as well.
How does exceptional strength work for damage?
your melee damage is your Str mod + your Super-Str level.
How do skill points work, if I wanted just some smart techie?
each skill rank costs 1 power point. if you want really high skill levels, one method is to buy Super-Intelligence (or whatever other attribute whose skills you want high). if you only want the high skill bonuses and don't care about the stat's other benefits, you can apply Flaws to your Super-Attribute so that it only affects skills.
alternatively, one common house rule is to simply grant more skill ranks per power point; 3 ranks per PP is the most common.