"Fun in all 4 Seasons" is the tagline
You can decide how long a season is and which types of weather you can get, like if you never want snow that is possible! Seasons have to be at least 3 days, but in the demo we saw they had put one to 128 days!
Saw diving board in action – but no screenie sorry 
If you hover over the season icon you see the current game temperature in celsius or Fahrenheit
Spray tan cost 90 Simoleons – but remember this is not in the final build yet, so that might change
Spray tan failures can be handprints all over the Sim, yellow or purple streaks, or a "half and half"
Spray tan fail!
Parasols protect against the sun
You can get different types of snowcones
Ocean can be cold to get into, so Sims may be reluctant to walk into it
Wearing your swimwear in winter makes you Sim get cold/freeze faster – similarly, wearing outerwear in summer makes your Sim get hot faster
With the athletic skill your Sim can swim further out to sea
Besides rollerskating there is also iceskating
You can have ranked matches with the new football (soccer) item
During a hotdog eating contest your sim can throw up, very charming!
You can have knee deep snow – when your pet walks through it you might just see a tail and sometimes bit of a bump
Plants hibernate, not die, in winter.
A new plant is the pumpkin that you can carve for the spooky holiday and can make pumpkin pie with
Supernaturals have their own interactions with the rollerskate rink, it does not invole rollerskates! Werewolves will do their "hunting" run, ghosts hover, and fairies will fly around the rink.
If fanmade worlds have unique custom made objects, it is not guaranteed weather will work perfectly with it, but worlds with EA objects should work. (Hopefully custom content creators will be able to easily update incompatible objects, but that's just speculation)
If you own Seasons, a small patch will probably update any Store worlds you already own and use to be seasons-ready. It will probably not be necessary to re-download Barnacle Bay, Hidden Springs, etc in order to have the weather work correctly with these, so you will not lose your saved games in these worlds. However, the members of the Sims team present at the Q&A session were not 100% on how this is going to work at this point, but they want to make it as easy as possible!
Likes cold/warm are new traits, but those are not confirmed as the final names.
Likes cold has less chance of freezing, likes hot had less chance of overheating
You can have summer festival items in winter, but the food you buy at the festivals are unique for each season