This is my thought. The term "key" is still kinda new to me, as back when I collected as a kid I'd never heard the term if it was even in use at the time, but as I understand it now, it's a standout book. First appearances of major characters, deaths of major characters, first issues of major runs, these things to me are keys. When the term gets slapped around too freely, it waters it down to the point where it starts to lose meaning. I've heard people use the key term for books that most of us would consider dollar-bin fodder. Oddly, with maybe the Batman exception, when I was growing up origin stories were almost on par with other major books, but this collecting niche seems to have almost died off completely. Is this because these characters have been retconned so much by now that their old origins don't even matter, or that with the interweb, we can easly read the origins of characters we like without having to buy GA or SA books to read it in?
Marvel Team-Up is what other first issues are, the desired book for a run collector. I'd like a copy myself, as I grew up on beater marvel-team ups from the quarter bins, but I don't think I'd go so far as to call it a key. Just my 2cents.