52 day year? Seems like it would work pretty well and not be excessively long. That's a 17 IRL hour year. I'd like to see different colored leaves and grasses.
IMHO season changes should NOT be bound to biomes, but rather be global and aesthetically pleasing; and affect percentages of various events, spawnings and timings rather than the absolute availability of certain resources. I also DO NOT believe there should be a "bad" season and a "good" season, but rather they should all have certain benefits and nerfs. There should also be calendars with a sexy GUI.
Winter:
Wolves, endermen, bats, slimes, and creepers spawn more frequently.
Winter has precipitation 18% of the time, 90% snow and 10% rain.
The color scheme should be white and desolate, with WHITE or BROWNISH DEADISH (not completely absent) leaves.
Crops grow at 25% the current speed.
Nights are longer.
You get higher level enchantments easier.
Few flint, saplings, or apples drop.
Moar Fog.
You lose bubbles twice as fast when swimming, and ice has a nasty tendency to form above you.
Ores are mined 45% faster.
Spring:
Leaves make a nice comeback.
Precipitation occurs 25% of the time (April showers...), only 10% snow, rest rain.
Flowers of many colors (bring May flowers) dominate the landscape.
Chickens, cow, and mooshroom spawn rates increase.
Crops grow at 160% the current speed.
Ores are mined 30% slower.
Passive mobs breed naturally (on occasion).
Villagers give less powerful trades.
Make tools break more quickly or nerf enchantments to keep it balanced, because I find Spring to be the most OP season in my suggestion.
Summer:
Sheep, ocelots, pigs, spiders, and squid spawn rates increase.
Precipitation 20% of the time, always rain (except maybe in Taiga and icy biomes)
Hunger depletion increases too for game balance and reflection of the desire to lay down and do nothing all summer
Ores are mined 45% slower.
Crops grow at the same speed currently set by the game.
Passive mobs breed naturally (on occasion).
Colors are much the same as that of spring, but maybe the sun could be more exaggerated and dominating.
Nights are shorter.
You lose bubbles only half as fast when swimming.
More apples, saplings, and flint drops.
Fall:
Horses(?), zombies, skeletons, and witch spawn rates increase (halloween).
Precipitation occurs 10% of the time, only 35% snow, rest rain.
Reds and browns mainly for colors.
Crops grow at 60% the normal speed.
Potion brewing rate increases.
Zombies siege more often.
Villagers give better trades.
Ores are mined 30% faster.
MISC
Oh, and how about we make directions more meaningful? This may seem biased to the northern hemisphere, but how about the further north you go, the more snowy biomes, ext hills, forests, and swamps; while to the south there are more oceans, deserts, mushroom islands, etc. Maybe put NEWS on the compass to make this change more noticeable.
For all mob spawn changes I would keep it simple as an increase of 60% and a decrease of 20-25% if not "increasing" during that season.
No season changes apply in Nether, End, or any similar dimension to be added.
Note that this is just my take on it, I think the idea has a lot of potential and I hope Mojang goes through with it.
There should also be commands. /set day [1-52] and /set season [1-4] A world should start on a random day (doesn't have to be start or end of a season), and each day will progress 1 on the 52 calendar. The vernal equinox (IRL March 20th) can arbitrarily be set as day "1" (season 1), and thus summer would begin on day 14, fall on day 27, winter on day 40, and spring would begin anew on day 53, which would go back to being numerically referenced as day 1. PLEASE DO NOT forget to make seasons a gamerule, just for backwards compatibility of maps and in case they are found annoying by members of the gaming population.
Season changes could be either gradual or dramatic. Ordinarily I would say gradual as would most I think, but dramatic changes are more of a shake up, easier to code, and probably fit the theme of minecraft better.
I put some thought into this (30 minutes) and I would appreciate upvotes or feedback.