The classes have been reworked to be balanced at all levels. One of the biggest perks in third edition is freedom of race and class. Gone are the days where dwarves couldn't be wizards, or gnomes couldn't be paladins. Gone are the confusing calculations of multi-classing. A word you will see alot is stacking.
Some bonuses stack, some dont.
One of the coolest things added is the heroic feat. They are little abilities which give you bonuses in other areas, no success checks required. For example, there is a feat which allows you to use your dex modifier as a to hit bonuse with certain weapons, or one that allows you to fire into melee without a penalty. Skills have been reworked totally. You will not recognize them except the names.