I agree, this is a horrible decision. I just spent an hour searching for the "archive" keystroke, and of course found none. I did find that "delete" as well as "command-delete" BOTH delete the message to the trash... what's the point of that? Two ways to delete and no way to archive??
@Sam Halliday you're spot on... I hate to say it, but ever since Steve started to become less involved, Apple has steadily been having more trouble "finding the plot." I just downloaded the latest Pages (5) and, OMG, what a horrible step backwards. Deleted it, sticking with Pages 4 and... sigh, Parallels, Windows 8.1, and Microsoft Word for any serious word processing.
But back on topic: For those of you fed up with Apple making Mail less and less useful, check out Airmail. I finally installed it and have been trying it out. It's pretty darned good. A few things I don't like but it works flawlessly with gmail, and most important... it doesn't indescriminately delete your mail!!!