One area that exception based design helps is this kind of thing. The fact that, there are chunks of the rules that lots of people don't
need to know in order to play the game is good. Generally, you learn the rules better by doing, than by reading, so people that interact with a rule a lot will eventually learn to use it. (i.e. if you have a power that pushes people, you'll eventually remember automatically about the saving throw people get to avoid being pushed into fire/off cliffs/etc).
I started DM'ing almost as soon as 4e dropped. I didn't start playing in someone's campaign until the group I was DM'ing for was already into Thunderspire. I basically learned by doing, and since I was interacting with the monsters, but also all the PCs in the party, I learned the tricks of the PCs for the most part quickly. I had the fighter's OAs and Immediate Actions in mind, the Warlock's PBS, Shadow walk and cursing, the warlord's action point effect and inspiring word, etc ... While I didn't play some of the monsters just right (overlooking or misreading some stat blocks here or there), I had the rules more or less down for important stuff, and the corner cases I'd adjudicate on the fly.
One of the players I play with was often running my DM PC for me (the leader), and would often play the other absent players as well (in addition to the 'present in body/absent in mind' wizard) so his grasp on the rules are pretty good as well. In addition, both of us like to try new character classes, so I allowed for some PC swaps between modules, which has allowed for other opportunities to learn.
The one group that I'm playing in, I find myself helping out the DM in terms of keeping track of the various lasting effects. It's not so much rules knowledge that is tripping him up (he
knows what a fighter can do when he marks someone, and he knows that my avenger discourages people from attack him, but he has a hard time tracking which conditions are on whom).
If tracking of conditions and the like are handled in a way that the DM isn't distracted by it, the rules shouldn't be as big an issue. However, it's a two way street where the player should at least know about his own conditions (on him and created by him). As a I DM, I try to track the various conditions on my monsters (next to initiative and current HP I'll put notes about conditions, or whether regeneration was disabled, etc) but I need the PCs to track their own stuff. I sometimes have to rewind to reminded saves (luckily, most save end effects rarely apply between turns.)
Ultimately though, rules rarely come up as a problem. We have have to look up specific skill usages sometimes, but that's probably the most time consuming. Generally, the less often a rule comes up, the less likely I'll know it off the top of my head. [Heck, the player that plays most of the characters has the hit location charts from MechWarrior memorized just from hearing us call out the number and the DM naming the location so often). An impatient player can be a great foil if you have a bad memory, as he'll remember the rules just to speed things along if you keep forgetting the same rule and it always eats up a bit of time to look it up
