It seems the words "key" and "rare" (separate GA discussion currently) get thrown around quite a bit lately. The interpretation of the words mean something different to each individual, so I'm not sure that there's there's a perfect line in the sand that differentiates a "key" from a non-key. Rare may be a little more quantitative, but "key" is certainly in the eye of the beholder.
I tend to err on the conservative (?) side of the word and use "key" for very crucial or pivotal events in comics (and I don't even consider all 1st appearances of minor characters as "key"). In my definition, I'm not sure many SA Aquaman comics qualify at all. I'd interpret the 1st GA Aquaman to be a major key, the origin of Aquaman (Arthur) to also be of significant importance, and of course the creation of the Justice League (with Aquaman) as "key". After that, I don't think anything else qualifies in my book. Of course, I'm also one that doesn't think the 1st appearance of Lobo or X-23 or even Ms Marvel as a "key".
I can see the merits of Showcase 30, and I get that it's a "key" comic for a lot of folks. I just don't subscribe to adding the word "key" to just about every minor event that's happened in 80+ years of comic storytelling. But this one is certainly debatable, I'll give you that.