It mainly defies fantasy common sense with Wizards, though, and they are the most likely to really benefit dramatically from that really high INT, and to have such a dramatically high INT in the first place. The part that does bug me a bit is that if you create a 20 INT Staff Wizard with leather proficiency at first level, you've got an 18 AC.
I've often seen a wizard, sometimes even a frail-looking one, block a mighty sword blow with his staff, usually resulting in a shower of sparks. Often it's done with great speed, but occasionally it's done with a sort of casual grace, lending credence to the "I knew where the attack was going to come from before you started your swing" school of thought.
Which is pretty silly IMO. A 1st-level wizard with an AC better than a first-level 2H-fighter, and he's a better spellcaster for that 20 INT too. It works in the game, so whatever, but it's at or near the top of the list of things that doesn't make sense about 4e.
I don't find it particularly odd that a wizard who specializes in defense can beat the AC of a fighter who specializes in offense. The fact that the wizard gets benefits other than AC from his high int is fairly well balanced by the fact that the fighter doesn't need to spend any ability score points to get his AC.
It doesn't mean it makes more sense with Warlocks or Warlords, though, granted.
Or any less, of course.
So now guys who are really weak, frail, and clumsy but got really good grades in school are at a defensive advantage in melee over guys who are really strong, tough, and nimble.
You've got a few problems here. Firstly, unless they've rolled their scores and gotten very unlucky, there aren't any characters who are really weak, frail, and clumsy. Using point buy, they're going to be average or better in at least two of those qualities.
Secondly, intelligence and grades in school are not as strongly related as many people think. Intelligent people can recall relevant information swiftly and accurately, and can draw connections and reach correct conclusions quickly. These are both useful qualities to have in a fight, but the speed involved is only sometimes helpful for school grades.
And finally, higher AC is not the same as better defended. Hit points and healing surges need to factor into that calculation as well, and a strong, tough fighter will be have significantly more of both than a weak, frail wizard. The wizard might be on an equal defensive footing, but he won't be at a significant advantage, barring unusual incoming attacks.
Hi-larious.
I know! I'm totally cracking up over here!