I think it would be a nice touch (maybe farrrrr too much work... especially for release) to include seasons that cycle.

Autumn - tree's shedding leaves etc
Winter - snow / cold / windy
Spring - lots of green / flowers / life
Summer - sunny by day / thunder storm at night

Now I know this sort of workload isn't very viable, maybe if we all expected ARR in 2014, however an alternative would be to add specialized weather effects that are associated with the seasons..

However just imagine say Coerthas, in Summer the snow would melt and it would have a much more 1.xx look, and in winter it would snow.. But yes workload isn't very viable.

An alternative would be to introduce weather effects that are only apparent during the specific season (leaves falling from the sky in autumn, sleet falling during winter etc). This way the seasoned effects don't have to modify the landscape, but would still get the feeling / impression.