I think we project a lot of personality onto characters in Fallout 1/2. A lot of them we don't even get to see their faces. Compare Ian (long hair, biker leathers, loves chems but wary of the stronger stuff, doesn't mind shooting people in the back). Not much else to go on with him, since he only gets a couple dozen lines of dialogue, if that. Sulik, Marcus, Vic, Tycho, et al, they all have interesting backstories, at least some of it. But then they don't really have much in the way of character development aside from their introductions, apart from a line here or there in specific circumstances.
MacCready, in comparison, has loads of dialogue, opens up more as you travel, opens up even more as you complete his quests (two of them, not counting his stuff in FO3 in lamplight) has a unique outfit and choice dialogue in certain events and conversations with others. And it's nice to see a returning character, even if they brush over the history with Little Lamplight and big-town (what a shithole).
So... I'm not sure about your points about the characters in comparison. FO3 of course they were just window dressing for the most part, especially your robot/baggage cart companions, but then they did have to worry about modelling a million different faces, outfits and voice acting. I can't remember many characters from the old Daggerfall game, for instance.
Writing has definately seen a big drop there, I do agree with that. Bethesda don't seem to like following up quests with satisfying endings. It's okay if they end abruptly, and then you get some resolution to them later, but you frequently do not. A lot of the Oblivion and Skyrim stuff ended like this: Quest - resolution - "thanks" - never mentioned ever again. Sandbox crap for sandbox gamers.
Oh, and New Vegas characters were awesome. Getting Veronica her prom dress (please ignore the bloodstains, hon', they'll wash out) and her reaction squeal was an epic moment that gave me man-proud for all time.