Constitution increases the maximum AP amount. You may be able to get enough from equipment bonuses (especially for ranged characters), but you'll probably want to at least boost it a little.
For intelligence, strength and dexterity, the maximum cooldown reduction is at 15; higher values can help with damage and the chance to cause status effects. An intelligence of I think 22 is enough to max the status chance at 150%, though there are debuff skills that can help with that, as well. I've seen a few comments saying various skills started to hit diminishing returns after 17 or 19, etc.
