The Wizard doesn't need massive defenses in 4e because, aside from the 3e Knight, 4e is the first edition where Defender classes can actually protect their allies (when there isn't a 5' doorway to stand in). Paladins and Fighters have high hp and defenses because they're soaking up the damage in lieu of the Wizard (and other squishies).
4e is well balanced. Everyone gets to save the day some of the time, and everyone needs everyone else to do so. The wizard's save the day abilities are nonetheless some of the most potent, often ruining the ability for entire groups of creatures to coordinate themselves effectively.
The Wizard's Daily powers are impressive. Daily powers are a very small subset of his used powers though.
As for Wizards not needing massive defenses, I guess that's true if your DM throws most of your bad guys at the Defenders, and never or rarely attacks from the rear or attacks from all directions simultaneously. Every Wizard I've seen, though, does the feat/power tax of taking defensive abilities to protect his butt because the game system doesn't do that straight up. Paladins and Fighter? Nope. The game system protects them straight up AND gives them good powers.
In my experience with DMs who challenge the party, it's pretty darn easy to take down a 4E Wizard if the bad guys are intelligent and the DM runs them that way. If your DM doesn't do that and allows the PC Defenders to lock down a bunch of bad guys most encounters, then yeah, I can relate to your POV here. That's just not my experience in both PBP and in home games. The Defenders tend to lock down 1 to 2 foes at most (average per round, some rounds are higher), everyone else in the party is fair game.
Example:
Int 18 Con 12 Wizard with AC 14 and 22 HP vs. Con 12 Fighter with AC 19 and 27 HP
First level foes with +5 to hit and D8+4 or 8.5 average damage, 12 on a crit.
It takes ~4.2 attacks to take out the Wizard. It takes ~8.6 attacks to take out the Fighter.
Granted, the Fighter will often get attacked more often, but during those rounds or encounters were the Wizard is the focus, he's boned. On rounds or encounters that the Fighter is the focus, he's not boned.
Against foes 3 levels higher, it's 2.5 attacks (which can easily be a single round to unconsciousness with decent DM dice rolls) for the Wizard vs. 4.6 rounds for the Fighter. Usually, the Fighter, even against higher level foes, can react to the situation. The Wizard cannot always do so, especially if he is unconscious.