The soul shard change for warlocks is a pretty big change. I play demonology and sometimes decimation goes away just before the soulfire goes out and yadda yadda, a few wipes and your soul shards get perilously low. It can be inconvenient, but that's not a huge thing. Finally having the same amount of bag space available as every other class will be nice, too. However, this is a small part of what will change for warlocks.

Removal of spell power is the bigger change. Warlocks really don't care about anything else. Spell power pretty much trumps all; haste, crit, and spirit are just tiebreakers. Warlocks used to stack stamina and be the hearty casters who could take a beating in a pinch. That went away in Wrath. Now to get spell power, we'll need to stack intellect - a stat we've never needed much thanks to Life Tap. Also, with spirit as a healer stat, Life Tap glyphs and Fel Armor will need an overhaul. How will Blizzard preserve the warlock identity when we're no longer forcibly removing the souls of our enemies and we itemize like that OTHER magic casting pure class? I suspect the important changes will be regarding this.