I like that there are only two armor categories in 4E. It's either heavy, or it isn't. Makes sense to me.
I never liked how 3E really only really had three armors, total: mithril chain shirt, mithril breastplate, and mithril full plate. Everyone pretty much just used the best armor for each category. Sure, Druids wore Hide, but that's the exception.
These new rules at least give some variety.
The armor-donning time is a bit of a surprise. I understand that the goal is to not allow swapping armor in the middle of combat. I guess it's true that D&D considers armor to be more than just a breastplate, jerkin, or hauberk: it also includes boots, greaves, cuisses, tassets, spaulders, helm, gloves, bracers, elbow and knee cops, and gorget.
Truth be told, it takes a lot of time to get into all that gear. I know; I've done it. *Can* you speed-don your armor? Sure, I guess it could be done. Do the Players Handbook rules need to address it, and bog down with rules for "hasty" armor, or half-armor, and so on? Nah.
I think the armor don rule, like the rule for cover and a few other rules, is best understood in the context of the complete ruleset. By which I mean the DMG, which we haven't seen. Mike Mearls has mentioned on a few occassions that the DMG is actually useful this time 'round, and gives specific guidance on adjudicating these edge cases, like if a player wants his character to take a couple rounds to throw on just his hauberk and helm and rush out to the fight without all his other protective gear.
In other words, I get the impression that the 4E PHB contains all the rules, but not all the exceptions. And that the 4E DMG empowers DMs more, and maybe advocates and more clearly communicates the utility of conditional modifiers.