Spring has definitely arrived here. The cherry trees are blooming and the swallows have returned from wherever they spent the winter. It has me thinking about haikus and seasons.
I have wanted to make seasons matter in my games back at least to the first ACKS campaign. Rivers freeze or flood, pine barrens burn, passes close, leaves fall and encounter distance gets longer, monsoon rains fall and encounter distance gets shorter. Rocs migrate, bears (and dragons?) hibernate. Seasonal festivals and harvests, summer warfare and winter hospitality, ports closing in the winter. There's lots of fodder for variations in gameplay. Better still, it's cyclical variation, rather than a torrent of novelties, none of which ever recur. What you learned last winter, you may get to reuse (if you live long enough).
I also think nothing says "This is not an adventure path, there is no cosmic threat that you are on a deadline to save the world from" like having a campaign sprawl out over years of game-time. I think this was a big part of my initial attraction to seasons in play. I want my players to be able to say, "Nah, adventuring in the snow sucks, let's just take the winter off and fool around town for a couple months."
Unfortunately, this runs into trouble if you have nonzero "background" cash flow. If you're in a situation where your net cash flow is negative, like having too many mercenaries or a DM who actually enforces the cost of living table, your ability to take time off is very limited. If you are in a net positive passive cash flow situation, like having a domain, then your DM will go looking for mechanisms to incentivize you to ever go adventuring again.
I like the idea of a campaign long and mellow enough that the aging rules actually start to matter, and you can use the Oriental Adventures randomly yearly theme/events system and not be stuck on that one theme for the entire campaign. Let the dungeon and the wilderness by tense and hurried; let civilization be unhurried, month-to-season-scaled, stable enough for the farmers (and consequently the players too) to think about next year.
When I proposed to turn expenses-over-time and income-over-time into one-time lump sums [mercenaries][henchmen][domains], I was aiming at letting PCs disappear from the table for long periods of time, in the open-table style. I hadn't considered that it would work for PCs disappearing from adventuring for a long period of time just to take time off in-world. But I think it would work.