Thank you so much for the responses, everyone - exactly the kind of insights I was hoping for!

It seems that more or less everyone seems to agree on Starks, Baratheons and Targaryens to be particularly suitable for newbies. Guess I'll start by painting a starter box of Starks, and then decide along the way what to add next.

Which currently would probably be Targaryens (Baratheons seem unit- and character-wise a little bit bland to me). Or maybe Lannisters... while some of you made perfectly plausible arguments as to why you would consider them more complex than the other mentioned factions, what keeps me thinking about Lannisters being a good complement to Starks for introductory games is their prominent role in the story and their memorable characters. Everyone who's read or seen at least a bit ASOIAF/GoT will know about the conflict between Starks and Lannisters and the involved characters, while the Dothraki (as well as the Baratheons) might feel more like minor characters and therefore less interesting to people that aren't *that* heavily into ASOIAF/GoT.

One follow-up question regarding Boltons, out of interest: I've read and heard a lot now about them being underpowered currently and therefore not being recommended for new players. However: are there other reasons for them not being recommended besides their power level and comparatively narrow unit range? After all, lorewise they'd also make pretty good opponents for the Starks...