1) For PvP, 50/50 is as good as you'll get, as neither attacker nor defender should have an upper hand, all things being equal.
For PvE, IMO I think I should even con minions 95% of the time, lieutenants 85%, and bosses 75%. Why the adjustment? We're superheroes. Superheros should be able to wade through minions left and right, even starting out at level 1.
2) Attacker should always hit, defender should not expect to be missed. If they attack has the max to-hit buff, this should represent them having the maximum ability to hit people. They should not miss people who have little to no training in defense (i.e. defense buff).
3) Defender shouldn't get hit at all, and the attacker shouldn't be able to hit at all. Personally a soft cap approaching this limit would be most appropriate. If someone has a max defense, it means they are so good at defending they could not possibily be any better at stopping attacks. They shouldn't be getting hit by any attacks, especially by someone "untrained" in attacking (i.e. they have to to-hit buffs).
4) See #1. A maximum ability to hit people should find its match in someone who has the maximum ability to defend himself.
In general, however, it just be should as easy to get +def from outside of your defense powerset as it is to get +to-hit outside your attack powerset. As it stands, if two people receive Fortitude with all other things being equal, the one attacking will always have the upper hand as Fortitude's to-hit buff is higher than its def buff! This is just the entire game in microcosm. Vengeance has the same problem, as does the leadership pool (maneuvers vs. tactics, disregarding AT differences), etc. Heck, there is no counterpart to Focused Accuracy. The other first tier power of sets with Focused Accuracy needs to be Focused Defense to cancel it out.
Now balancing debuffs...ick. Not going to touch that. Hugh defense debuffs in rad blast versus tini tiny to-hit debuffs in dark blast for starters... (though I'd love a -37% to-hit in Nightfall to counteract Irradiate
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