Puddles would be easy to mod in, but hard to do well. You would need a LOT of activators, and frame rate would potentially plummet. I'm going to guess that we won't see dynamic puddles for the same reason.
dude what. i mean DIFFERENT GAMES HURR DURR but RDR did that [censored] flawlessly. even without puddles, tons of games have dynamic wet surfaces in rain - GTA4 has them, STALKER has them, and i'm sure there's a lot more but i haven't played any other modern games with a dynamic weather cycle for longer than five minutes (European fantasy sandbox RPGs bore me to tears).
stop thinking in terms of Oblivion modding when discussing whether or not something is doable in Skyrim. this isn't about PORTALS or WORLD SIZE or whatever the [censored] this is basic shader tech that has been in games since at least 2007.
i'm hopeful for seasons. i mean they have DYNAMIC SNOW this time so it would very easily solve the issue of HMM IT'S SNOWING BUT EVERYTHING'S STILL GREEN, MY IMMERSION HAS BEEN SHATTERED, and they've built their own foliage tech this time around, and considering it's probably going to have to respond to a) being cut down and b) being set on fire it seems trivial to script trees to drop their leaves once Last Seed rolls around.
also, fun fact: Oblivion was apparently going to have holidays at some point but they scrapped the idea. i don't actually remember reading about this at all but there's a ton of strings in the game settings named after holidays. it's kind of neat! i hope they manage to implement them in Skyrim.