Personally I think that the favoured enemy bonus should be applicable to all creatures, regardless of whether they are crittable or not. Reasons why? Strength damage bonus always applies. Point Blank Shot damage bonus applies. Weapon specialisation damage bonus applies. Bardic song moral bonus to damage always applies. All add a straight +x to damage.
The only thing that doesn't affect crit-resistant monsters is sneak attack and there are two observations there - firstly, it is lots of d6 damage rather than a few points (game balance issue) and secondly it is about striking at vital organs which are not there in these creatures (flavour issue).
I can't imagine why the D&D designers decided to arbitrarily weaken a restricted ability like favoured enemy (and for them to say "oozes are a suboptimal choice" seems as sensible to me as saying "the halfling outrider isn't supposed to have a BAB"!).
So that's why I think ranger favoured enemy should add to damage against *any* enemy type chosen. It's hardly going to break a game, and is it any worse than the fighter with point blank shot, bow specialisation and under bardic encouragement who gets +4 damage against every ooze, undead and construct he shoots at? Assuming they are vulnerable to piercing weapons of course!
Concealment/Cover
Regarding the concealment possibility: I'd got in mind Batman (I'm afraid) - the idea that this character is really elusive in his chosen terrain, hard to spot, he's always slipping sideways into the grass, hiding behind the scrubby bushes. The rogues find it difficult to sneak attack him on his own terrain but not because of uncanny dodgying, but because he always seems to be standing behind something that makes him that bit harder to target properly.
It is something that hasn't been seen as a class ability, and the fact that it is limited to a particular terrain means that it is not always cropping up (although it makes a better NPC ability in many ways).
In many ways I think it is better to allow additional terrains to be chosen as the character gains levels, because by the time you are 10th level you could be in very, very different places to where you started off and it seems strange to give class abilities which suddenly are not usable... everyone elses class abilities are really quite broad - even the druid, who is in other ways the most focussed of them all yet his trackless step is universally applicable.
Perhaps if you allowed him a favoured terrain from your schedule, and each terrain he gets, he has a 10% concealment/miss chance when being attacked as long as he is not denied his dex bonus?
Cheers