Among my two complaints about feral right now; yes I only have two, well maybe three. Anyway the one that bugs me most is using rage, and worse, energy to fuel a timely interrupt. Here I am happily smacking away and boom I need to interrupt in the next second an a half. If I don't have the rage or energy already available it ain't happening.
While tanking something where I know I will have to interrupt I can make the additional effort to keep a little rage in reserve for a timely interrupt. It actually isn't that big of a deal, just a matter of not hitting maul that one more time. Bear rotation remains relatively constant when it comes to rage usage per unit time.
Cat form is a whole other story, energy levels are constantly doing a roller coaster from near full back to nothing and that fluctuation more often than not will never line up for a convenient interrupt. Best case scenario you have the energy and it doesn't muck up your dots/roar, worse case scenario you just threw off a ferocious bite or something. I don't really like to consciously think about the numbers, but when you get into the cat groove you get used to having a certain amount of energy to work with, setting some aside really isn't a consideration. Saving some short term for a known interrupt is probably the best of both worlds, but that isn't always possible. Oh that reminds me, there are four things I don't like about feral.
Taking a look at other classes we have interrupts again on cooldowns, but using a more static resources like mana, focus or runic power. Even rogues don't seem to use their energy in quite the same way as cats do, so the occasional kick isn't as much an inconvenience; could just be that it is cheaper too. Many of these interrupts aren't even on the global cooldown. This is how I like my interrupts, yes I hit the button quickly, give me a cookie. Ideally an interrupt should have no impact on other abilities and having it on it's own cooldown is sufficient in my book.