Firstly, the tech demo showing the power armor and its usage was likely "cheated" just to show us stuff, or maybe just jumps to different points in saves to show us.... so by all means, power armor shouldnt be early way in the beginning.
Personally though, I think power armor is going to be Fallout 4's equivalent of your "Mount" like in the other Elder Scrolls games.
You wont get it right away, has maintenance and upkeep, and will likely will have to be left behind, like a horse, when you enter areas that "arent allowed"
How would this work? I Would think since its power armor and so huge, you could climb out, "lock it up" and come back to it later. Its too large and heavy to be stolen and carried away..but COULD be attacked (like your horse)...
Could also rule out theft by piloting, since not everyone can use them.
You would likely start collecting "parts" pretty early in the game, which would give you plenty of options when it comes to size, speed, protection, armament etc etc. (hopefully painting them is a thing, one of the chest pieces had "Flames" painted on em.
As far as armor goes, you see examples of during combat scenes, your actually BLOWING CHUNKS OF ARMOR OFF the hostile creatures (like a big orc)... So no only would this mean, if your hammered enough, part of your armor is gone, you now have a hole to be shot through... AND... thsi could be a way of collecting parts... Mutants and raiders wearing parts of the power armor like normal armor (just slung on them in random places) and thats how you collect parts in some way..
I would think that perhaps you could get air delivery at some point..like Skyrims magic "whistle for your horse" thing...If you left your power armor someplace.. perhaps you find a working virtibird and pilot, who you hire to find your suit and bring it to you..things like that.
I bet also, that due to its size, they might impose restrictions on inventory management...IT can only use certain weapons.. and collecting loot involves hopping out, pick [censored] up, then back in you go.. again, alot like the horse combat in Skyrim.
All in all, another awesome thing to build towards, long term goal to have ingame..along with the settlement building and such, collections etc etc..lots to do aside from teh main storyline.. which BE has always been great at doing!