I miss seasons. It added another level of immersion and fun that you can't really get with other packs. Weather is so apparent in everyday life, and it just seems absurd to play without it; a lot of players live in places where it's rainy or snowy most days out of the year. Plus, the atmosphere is just wonderful!
They really did weather right in TS3 Seasons. I loved the rain; you could see the rain fall in drops from the roof, and your sims could jump in puddles, or be struck by lighting and die, it was just really cool having that extra layer in there! But for me, its more like a "foundational" layer!