My favorite part about Seasons is definitely the holidays. I really love that you can make your own holidays and customize the traditions as well. I also love the new CAS items. The jackets and coats are so cute.
My least favorite aspect about Seasons would have to be the thermostat. I don't know how it works. I was very excited for it but honestly it is very annoying. So far, I've used it twice and it makes the bills skyrocket like it's nobody's business. My sims usually pay $10,000 in bills (they live on a huge lot) and I was fine with that but I used the thermostat to turn the AC on for like 2 days and my bills went to $40,000.
It's honestly worth it. At first I didn't see why everyone wanted this expansion out of others (personally I wanted University) but now I'm in love with it. Definitely one of my favorites. It's added so much dimension to the game.