Let's start with the new classes:
Beguiler: For everyone who ever wanted to play Morgan Le Fay, The Beguiler is a specialized caster that makes for a perfect BBEG. Their abilities off the battlefield are almost as potent as their abilities in combat. You could call it a bard with a full casting progression, or a wizard with naturally deceptive tendencies. whatever you call it, the game is a better place for the Beguiler.
Dragon Shaman: Dragon Shaman are... well, they are hard to describe. On one hand, i respect them as a class, they are effective in a wilderness setting (fast healing at first level is always helpful...) but at the same time, they don't really fit a defined niche. They have cleric combat ability, Great fort / will saves, but the only thing that really stands out about this class is how it best fits in tribal / dragon-heavy campaigns.
Duskblade Full attack progression, armor gained as you level, half spell progression, with spells that encourage you to mix it up in close combat, and a quick cast ability that works like quicken spell without the level adjustment. Plus, their spell list rocks.
Knight Not just another name for a fighter. Knights are everything any player could ever want in a meatshield. d12 hit dice, heavy armor class, full attack bonus, a GREAT will save (much to the surprise of everyone who tries to "charm the dumb fighter") and the ability to coerce enemies into targeting them instead of, say, the wounded and bleeding halfling wizard over there. In exchange for these benefits, they have a minor code of conduct about being honorable combatants.
Secondly, the expanded classes features.
I don't know about you, but i'e had a lot of GM's who house rule even the core classes because they "don't like the flavor" or whatever. That's what the Expanded classes section is for. Don't like a paladin's mount? Charging Smite for the ability to inflict +2 damage if you personally charge the enemy and hit him. Hate the ranger's animal companion? Now they can do attacks from range that count towards flanking bonuses.
Third, the new feats.
Okay, most of these feats do not have an immediately understandable benefit. But most of these feats are for higher level characters. all are pretty wicked ways to play. (Ki blast turns any Monk into Ruy, or Ken)
Fourth: New spells
Well, they fixed polymorph into a subschool to reign in the flagrant polymorph abuse... and they added quite a few nice spells for any caster, but the same could be said for anyone else. I think the best addition is the third level spell "alter fortune" for 200 xp, you force anyone to reroll... though Dimension shuffle (5th level) is great for breaking up enemy flanks.