I think that all the seasons should have no effect on actual block placement, but textures may change depending on the season and weather (But snow isn't placed in any biomes that shouldn't have snow in them).
In Spring, everything looks default. (Spring should be the default season in my opinion, since this is the time of year that isn't too hot or too cold.)
In Summer, grass and leaves look more saturated and green then they look in the game right now. It will also rain in snowy biomes, but snow on the ground will not disappear.
In Fall, leaves turn yellow/orange (Except maybe acacia wood) and grass turns back to the default color. (In addition, whenever it rains, some of the rain particles near sea level will be replaced with small leaves. I say sea level because I don't want leaves spawning in the sky.)
In Winter, leaves turn white, (Except acacia. Savanas should stay the same throughout the year in my opinion.) logs desaturate just a bit, and grass turns just a bit bluer like in Extreme Hills biomes. In addition, rain particles are replaced with snow, but snow blocks are not actually placed.
Because all that has to be done is changing the texture of a block, we should be able to avoid lag that comes from excessive block updates, as well as avoiding making too much of a mess for people that don't like snow.
As for 90 days, I think this should be a bit shorter. Acording to this chart, a 30 day month is about 10 hours, and if someone has about 1 hour of playtime doing Minecraft every day, that's one real life month before you can actually see another season. I'm sure there would be some cases where someone plays through a world way too fast to actually see seasons. (Although you would see each season pretty often on a server, considering 3 months is just a little bit more than 1 real life day.)
Also, should there be items like the daylight sensor and clock for seasons?
But anyway, I support the idea of seasons in Minecraft, even if other people think it's dumb. I feel like it would make time in a Minecraft world feel a lot more real, and a bit less like a every day is the same.
I get offtopic a lot, be warned. I also often mention a Minecraft clone called Minetest. The clone is nowhere near playable with it's default game in my opinion, but it's at least the start of a good open-source voxel rendering engine, and maybe one day it might be decently playable.