I'm really excited about Seasons! I'm pretty new to the Sims--just started playing a few months ago with Pets, but the idea of seasons just seems wonderful. For awhile I was torn between this and Supernatural, as I can't get both, but Seasons really seems better suited to what I want to do in my worlds and how I play.
I'm particularly excited about swimming (I was really puzzled when I first started playing to discover Sims couldn't swim!) but the rest of the seasons stuff--weather, festivals, etc, look really lovely, and make it worth it to me.