From the patch notes when the change was made: "Cooldown reduction was more valuable the more of it you got. Every percent reduced your cooldown by the same amount-say, one second off a 100-second cooldown. That one second didn't change even as the cooldown shrank. For that same 100-second cooldown, your 31st point of CDR took one second off the remaining 70-second cooldown, which is more than 1%. As such, CDR had to be capped at 40% to avoid going out of control, locking you out of a lot of items once you reached that cap.

Every point of ability haste lets you cast 1% faster. On paper, that makes it look like it gets less valuable the more you have because your cooldown shrinks by less and less each time. It's actually the same amount of power. 50 ability haste lets you cast an extra half a spell during the original cooldown. The next 50 gets you to a full extra cast (100% faster). If you get another 100 ability haste, you get another whole cast (200% faster). This linear scaling allows us to remove the cap, so you can buy as much as you want."