After playing the Beta for a few hours with each class, here are my impressions on the ability cooldowns that I've experienced: Grenades & Melees: I think the cooldown is too long. Especially with some of the changes that I've noticed (how many grenades and melees do less damage), I'd like to see the cooldowns a little faster so Guardians are slinging more abilities. This is Destiny after all, and the point is to be super awesome space wizards with abilities to boot. If I wanted a shooter sans abilities, I'd play most any other one. Since they do less damage, let us use them a little bit more. Class Abilities: I think these are doing okay. These shape the world more and provide good boosts to your entire group, and I think they're at a good place. Often they're used for panic situations or to strategically lay siege to a location, so they shouldn't be available 24/7. Strategy is the key here, which brings me to supers. Supers: The cooldown seemed very long to me. I don't think that supers should be plentiful. We're space wizards with abilities, not gods with supers. I do, however, think that the cooldown should be increased at least enough to provide well-playing Crucible combatants the ability to have more than 1 super. If you're doing horrible in the Crucible, 1 super should come 2/3 of the way through the game. But if you're doing really well, you should be able to get a super a little before half way through. This would create a more interesting strategy where you have to decide whether to save your super for a tide-changing moment and know you won't get the second, or risk using your first right away to immediately change the tide and risk not doing well enough to get the second or not having your super at a more crucial time because you already used it. This length of a cooldown would also make it more plentiful in PvE, which I think would be welcome. Sidenote: orbs don't seem to give as much of a bonus. This faster base cooldown could allow the orb bonus to stay where it is and still be okay.
Forgot to mention: implementing the suggested cooldown change for supers would also make it so you don't have to feel like you're only getting your super in the last 30 seconds of the Crucible match. I don't like how it can feel like a panic rush of everyone using their supers at the last minute when often the game might already be decided by that point.