I'm very much in support of as much weather as the physics and graphics can gracefully handle. And I definitely think it should affect vehicles or maybe even infantry in real ways. If a major offensive is in the planning, a weather report should be a necessity (though in particular areas most especially). If your faction doesn't possess a weather station, you'd have a potential tactical disadvantage, arriving to inclement weather you aren't prepared for.
Snow storms: Low visibility, decreased range thermal imaging (if there is such a thing?). Aircraft would ice up if flying into a storm front: performance slowly decreases and if there gets to be too much buildup, the plane can't fly anymore and will crash. You can fly above the storm on the edges but the center would be full area denial. Perhaps infantry will start to freeze up, and will take damage if they are outside in the storm for long, say a couple minutes. I can imagine tense battles in near whiteout conditions, fighting from wind-break to wind-break, and you know the chances or reinforcements making through are slim. I hope that snow and ice has reasonably dramatic effects on the handling of vehicles. Skis would also be an interesting addition, and screw anyone who says 'leave the skiing up to "tribes" series', they don't need a monopoly on skiing in fighting games, it could still be hella fun. If real snow was attempted, you could cert. a plow for your sunderer too.
Sand storms: Lowered visibility. Damages aircraft if they try to fly in it. Vehicles could have their filters clog and engines overheat, slowing movement while it cooled. I remember pictures of some incredible red sand storms from Australia from a few years ago, the skies the color of blood or tomato soup or something. That might be impressive.
Rain: If the engine can plaster all the vehicles and characters with a nice coating of mud, they should add some rain to the world. Plus I'd like to see something clean up those dusty skies for a while. A real cloud-burst downpour in the jungles would be cool too. Or a dense layer of fog covering an early morning battleground, the sound of boots crunching and tank engines roaring from enemies hidden beneath the veil.
A similar environmental effect that I was thinking about
: wild-fires. The ability to light up the grasses on the prairies added an interesting flair to 'Far Cry 2.' If their are any grassy areas on ps2 continents, that would be kinda cool. Use the fire to direct the enemy, hide under the smoke to avoid air-support. Once the next good rain arrives, the whole place regrows anew.
Weather is good. +Support.