I found nothing useful with search, and NinjaRooster said on the "general recommended ideas" topic that there are no in-depth threads he or she could find either, here are my ideas:
Spring: Spring is probably the closest season to normal indev and most of infdev. Nothing special here, except lots of flowers, lots of leaves grow on trees, and plenty of peaceful and enemy mobs. It rains about once every 5 days in this mode.
Summer: The grass in summer, if far enough from water, will brown and dry up, making it flammable. Water levels will recede 1-2 blocks. Larger percentage of peaceful mobs. Very little rain.
Fall: In Fall, all leaf blocks on trees turn various colors, then fall like sand/gravel. Higher amounts of enemy mobs. Leaves fall when near trees; purely cosmetic.
Winter: All exposed blocks get snow on top of them; Fires will melt snow into water AND put out the fire. Trees lose their leaves, and the uppermost layer of water turns to ice. Very few mobs. Snow particle effects.
Ideas for seasonal mobs:
Spring: Rabbits. They would appear normally in the majority of the year, but in spring they breed like, well, rabbits. This results in a lot of the little things swarming about. They eat plants and grass, and don't attack.
Fall: Tree monsters; can be differentiated from real trees in that their leaves are not affected by the normal fall effects. Fairly rare. Only attack if attacked first, but they pack a punch.
Winter: Killer snowmen. They look like snowmen with blocky bodies and attack similarly to zombies. They melt into 2-3 squares of water if they stray near torches or other heat but are tough to beat with conventional weapons.
I don't know where you live, but here in Arizona, where it's hot ALL year long, looking outside, grass does not dry up and turn brown. That only happens during droughts. Plus, it would look hideous if outside of your house was a bunch of dried up grass.
Seasons are so straightforward you don't even need a discussion topic. The only reason you would is season specific mobs/resources which you don't end up addressing.
PLUS. Fire doesn't occur in winter?
No offense, but if a topic is lacking an indepth thread, it helps to make an indepth thread. This is skimming over the basic effects of seasons that people are familiar with, and a couple of quirks that are thrown out and not analyzed.
Basically, I'm saying that 2 lines for each season is not in-depth.