Brood War is the shit. Still my favorite expansion pack. I'm hoping Heart of the Swarm is the modern day Brood War.
I really, *really* like the AI adjustments and tweaks Blizzard has done to playing the CPU in the HoTS beta. Played a bunch of 2vs2 cpu with a friend, and the game auto adjusts levels after each victory or loss. You can see your difficulty progression via a bar from easy to brutal. Each difficulty has varying levels too that the game will tweak based on your performance.
AI is much more vicious now. Easy is still fairly easy, but moving into medium and definitely by hard difficulty the cpu starts using more advanced tactics. The ai will properly team up together, retreat and regroup if needed, or take out your expansions before attacking your main base. We could handle Very Hard and Brutal difficulty alright in WoL, but when we moved from Medium to Hard difficulty in HoTS beta we got clowned.
The XP leveling system is a small bonus, but a nice one. Those of us with leveling OCD will want to switch between using Protoss, Zerg, and Terran to get those individual levels up. The only rewards outside of extra portraits every so many levels, but there is just something about a post match progress going up that keeps you move engaged. Fucking Call of Duty.