It looks like haste is an even bigger stat than ever for 4.0. We are casting a lot of direct heals, now, and more haste means they land faster. In particular:
1. Regrowth is now our top-up spell, and haste speeds it up until a cap of 50% where it has a 1.0-second cast. Faster regrowths on a tank mean it has a better chance of landing before the next melee swing. Faster regrowths around the raid mean we can top up more people after a decimate before they die.
2. Healing touch hits like a truck now. I’m seeing it land at twice a regrowth and three times a nourish. However, the cast speed is just sluggish enough that it’s difficult to use well. Getting it’s 2-second cast time lower makes it much easier to use. Haste is valuable for this purpose until healing touch is at 1.0 seconds, somewhere around 150%.
3. Rejuv and regrowth have better throughput when hasted, which is especially relevant for tank healing. This is useful up until stupid levels of haste. (Instant rejuv? Yes please.)
As much as it makes sense to talk about a soft cap, it's around 50%. However, 50% is already nearly impossible to attain at level 80, and it should be even harder at level 85. For practical purposes, resto druids have no haste cap in 4.0.
So how much haste should you get? Right now, mana regen seems to be fine for most players. Many are even spamming regrowth and not running out. So don't worry too much about regen.
What about throughput? To the extent you really want to worry about raw throughput, haste should be balanced against intellect, crit, and -- if you tank heal -- mastery. Right now, though, throughput is pretty high, and for many healers it is probably as high as matters for the raids they are doing. In that case, it's more important to get heals landed than to make them bigger, and landing heals faster is all about haste.
TL;DR: Haste is great. Get all you can without going OOM.