As a combat simulation, 4th edition is a rich, well-balanced game, in ways that 3rd edition really isn't. It's designed to be played as such - and it's fun when played as such. The DM is encouraged to treat the game as a game, and fudge things in the name of player fun. It is more fair that the high level wizard and the fighter have an equal number of combat options at their disposal. 4th edition has this.
3rd edition is more of a world-simulator, it is less focused on combat and player-experience and it's combat elements were much less compelling. It is more realistic that a high-level wizard would have many tricks that a simple fighter can't match, and 3rd edition has this.
The DM's role is more or less the neutral, uncaring referee, and the content is not tailored to the players.
I prefer 4th edition overall.