The basic problem I have with Take 20 is the fact that it applies to every skill that can be retried without penalty for failure, and simulates the player rolling often enough to finally hit 20. So basically it took some bad habit that arose from metagame thinking, and made it a core rule to shorten time...but doesn't apply common sense to the situation at hand. Because basically what happens is that a rogue is searching a specific 5 foot square
over and over and over, even if he didn't find anything at the first try. Which, at some point, simply sounds extremely comical, if you think about it...a rogue sitting in a dungeon, staring at one 5 foot square, mumbling "but there MUST be something, I just have to look hard enough".
I know it's nitpicky, but I wish they simply had made Take 20 the aforementioned "Take your time" rule, and stated that it simply means the character being extra careful, slow and methodical, etc. It would have given it all a different flavour altogether, in my opinion.
Take 10 is not so much a problem, since it implies the character is using his "average" amount of competence in trying to overcome an obstacle, and if that's not enough, it simply isn't enough, and he has to try harder. The problem might be that the 5% difference that come with using a d20 may look small compared to the 50/50 chance he suddenly has of not making the climb check (for example), and the 25% chance he has of falling, while the other guy takes 10 and looks surprised his buddy can't keep up...but otherwise, we'd have to use a d100, and even there you can get moments where a tiny difference can give you a huge competence gap all of a sudden. So either we need to eliminate the Take 10 rule, modify it (Take 5, and change "Average" difficulty to a DC of 5), or live with the fact that people whose average performance isn't enough to overcome a lots more than average challenge face a real and threatening chance of failure. After all, a Climb check DC 20 is not Average (DC 10), but Challenging, and something challenging should be average only if you are REALLY good (Skill bonus +10, for example, is pretty good, since you have to be 7th level to get there in the first place for a class skill).