Extremely bad for me I may say.

For example, exactly like in FO3, I may want to play a damn ninja with a stealth suit and kill everyone silently, snipe them or with a sword. I may then replay the game as a mad scientist or whatever with a lab coat, robots and laser pistols. Then I may replay again as an explosives nut killing mostly everyone with grenades and my trusty brahmin prod. Etc Etc...

I could do all these in FO3. Having to wear power armor every now and then for some missions not only forces me to play with the same way in every replay, it also ruins my RP. So unfortunately it is very bad for me and it is one of the things that are not easily circumvented with mods.

And how exactly is having to use power armor against a powerful enemy stopping you from rping as any of those things?

Alduin required dragonrend to beat; was that ruining the RP of someone who didn't want to use it? And who's fault is that? Probably the person actively working against game mechanics

I'm trying to think my way back through FO3 and FNV, and I can't really recall a situation that "required" you to fight, navigate, or defeat a specific enemy using certain "category" of skills, weapons, equipment, etc.

I can think of a couple spots where a certain skill set, build, or gearing made it a little easier to deal with some obstacles, but it hasn't ever been BGS's way to lock down a specific path on an encounter and declare, "Thou shalt use stealth and only stealth to defeat this enemy."

There were a few scripted mission events like the Behemoth fight at Galaxy Radio where everyone screamed "use the Fatman" but I can even recall one time where the fatman was under a pile of flaming wreckage and I couldn't reach it so I ended up killing the thing with a combat shotgun and a LOT of shells.

Where did you hear about the branching in the quest from? This adds a little more hype for the train!

Definitely going with it being early on and for familiarization, myself.
If this is anything like the last two fallout games, we will need to move towards more of a later stage area, from a ways out.
I think Sanctuary Hills may be a development in the surrounding area of one of the Mass Audubon wildlife sanctuaries.
This could be pretty close to concord, where we find Minutemen National Historic Park, depending upon the sanctuary.

If you are set on rping many different types of characters, then you should accept that fact that different characters have different limitations. I doubt anyone will be required to wear PA, but if you don't, then you should expect that some areas are more dangerous, and maybe even impossible, without the proper equipment. For example, if you playing a nerdy scientist type, you shouldn't be expected to be killing stuff left and right anyway, even mole rats should pose a challenge. Given, the original FO games made it much easier to RP non-combatant types, there would still be stuff you couldn't do. Want to play as a charismatic speechy smart guy? Well, then don't run into a deathclaw cave, as the outcome would be your death.

Personally, I hope that this is how FO4 is like. I hope that deathclaws are at least as deadly as they were in NV, I hope that decisions on my character build may rule out certain paths in the game. It makes the game far more engaging.

It's a whole different story to have to use a complementary skill once in the game and another thing to be forced to finish the missions in the power armor. I don't want to work against the game's mechanics. I just want to do what I did in FO3 and FNV, I don't think it's too much to ask.

Some limitation are natural but as in a real life situation, the perfect sandbox rpg will offer you several different ways to go through a situation. With a weak character ofc I don't expect to be able to handle deathclaws face to face but I also don't want the quest objective to be like this: "Get in your power armor and pwn the deathclaw!". Imagine a portion of the quests being like that.

Doesn't that only matter if the quest is mandatory? Playing specific roles usually means there will be quests you skip, as they don't fit your character. Yes, multiple ways to approach a quest is always best, but not every quest should be like that, which when playing a specific kind of character, you just need to know "well, this isn't something my character is made for so best course of action is to skip it".

This is something we do all the time, obviously there is stuff in RL we are not qualified to even try, and no multiple ways of doing it is going to save us from failure, so we just don't do it. We know our limitations, as would our character know theirs.

So, isn't skipping getting in PA and killing a deathclaw an option?

Power Armor: I seriously doubt we'll ever be forced to do it, and I'm not sure what kind of RP would absolutely refuse to step into a suit of power armor in the first place - it's not like your Ninja character suddenly isn't a stealthy ninja for knowing how to operate a suit of Power Armor, and stepping into what is basically a vehicle. Just like it doesn't break RP for a marksman with no interest in melee combat to pick up some passing knowledge of melee by reading a melee skill book (or in Fallout 4's case, getting a unique melee perk from reading an issue of Grognak the Barbarian). The quest with Preston Garvey in particular looks like it has multiple means of completion, too, and we're missing several parts of it to begin with.

hardcoe mode: I doubt it, and definitely not in the same form that existed in New Vegas. I do think (and I'm mostly just guessing) that food and water aren't useless in the base game this time around, though.

Modding: They said the Creation Kit will hit PC in "early 2016", and after that they'll open up modding for the XB1. And after that, the PS4. We don't know anything else, I think.

Integrating mods for 3 & New Vegas: It's impossible to tell when Bethesda directly draws inspiration from the modding community, and when they just have the same ideas. I doubt Fallout 4 would have lacked iron-sights and sprinting if those mods didn't exist for the last games. And Bethesda will almost never name specific mods that inspire them, but they do pay attention to what happens in the modding community.