I have to disagree with you on one point, the other four a certainly valid if for no other reason than hardware limitations, however on point number 4, I don't think that is going to be the case. As far as I can tell, with everything Microsoft has announced regarding the Xbox One this year, they seem to be trying to entice the PC gaming crowd to, at least, also be Xbox One gamers. If that is the case, then they are going to have be as unrestrictive as feasible, given the aforementioned hardware limitations. This also means making the mod development and publishing process as simple and easy as possible, which means being able to just port the mod over rather than having to develop from the ground up. It would seem possible given the ability to create universal apps for PC, mobile and Xbox, I would imagine the porting process to be an extension of that software philosophy(Of course it could natively support the mods as long as they are properly published to the Xbox Store, with certain restrictions, of course. Who knows, its a long shot, but then again, who would expected mod support like this on console in the first place). As of right now, the only size limits that I have heard of, and if anyone knows of any others, please just say so with links, is the save game files for Xbox One have a hard size cap, beyond that, I don't think Microsoft put any size limits on additional content for the Xbox One, or if they did, it certainly much, much larger than the 2 GB DLC size limit on the 360. But to be honest, I think they have pretty much done away with size limits with the Xbox One.
One other thing I have to disagree with you on, and that is whether or not Sony is going to have mods. I have my doubts, because I don't think that the current iteration of the PS4 OS has the necessary framework to support mods. To be fair, I don't think the current iteration of the Xbox One OS does either, as it was designed from what makes sense from a corporate standpoint, not a gaming standpoint(Look at the Xbox One reveal for evidence). As such I suspect it would take a massive overhaul of the PS4 OS, like what is happening with the Xbox One, to allow mod support and I don't think that Sony has the drive, or interest to do so at this time. Of course it could be coincidence that the big OS update for Xbox one is coming the same month Fallout 4 is scheduled for release.
Just one last item, and this is to add to the list: temper your expectations as to how well mods are going to work. Unless I have missed something, mod support testing looks like it is largely going to be internal, if at all. If that is the case, we are all going to be a part of one great big experiment, and stuffs probably going to be broken for the first little bit. On paper, it would seem things would go well given how similar the Xbox One OS is going to be to Windows after the update, but never trust how things on paper sound, ever, ever, especially when it comes to computers.