I don't tend to hold Fallout to a terribly high level of realism - this isn't a hard science fiction setting anchored in in-depth scientific extrapolation a la Arthur C. Clarke. Rather the setting is defined more by it's visual style and iconography.
I'd wouldn't mind seeing some areas with vegetation - but less because I'm concerned with realism in Fallout, and more because I feel I'd enjoy more variety in the locales I visit in-game. Greenery isn't terribly high on my list of priorities, however.
Verisimilitude is another matter, however. It's a standard gaming concept that there's more going on in the world than what is rendered - as fidelity in these games improves, however, that gap becomes more and more noticeable. I didn't care in Fallout 1, for example, that I could buy a squirrel-on-a-stick yet not see one in-game. With modern computing power, however, I'd expect to see some more wildlife beyond just the creatures that are going to be important in terms of gameplay. (Skyrim, for example, featured a variety of wildlife, and most contemporary games will include minor wildlife beyond just what's going to attack you or be hunt-able.)
I sort of feel the same way about weather, really - in past titles I could assume that there likely was some sort of (post-nuclear and twisted) weather pattern going on in the setting - but it was just never portrayed. There's nothing in canon that says it never rains in the Wasteland, for example, and a dust or radiation storm could be an interesting thing to play through, I think. But again, I won't be terribly bothered if there's no weather in the game. Though if I remember correctly, there was snow and snowstorms in Skyrim (can't remember if I ever saw it rain - I'm thinking I didn't but could be wrong.) So at I guess I think it could be within the realm of the possible with Fallout 4 to see some weather effects - and given the setting that could have some interesting outcomes, potentially.