I have to say that in coming from Sims 2, I had a lot of expectations about things that didn't really play out quite that way.  If you're a Sims 2 player moving to Sims 3, you've got to look at it with new eyes, so to speak, because there are things that are just different but might easily missed when working from Sims 2 expectations.  Here's some of the stuff I discovered so far!

Muffins Are Meat

What's that you say?  Toddlers make meat muffins in their tiny play ovens?  They sure do.  This makes me look at my neice's Easy-Bake oven with something of an eye askew.  I mean, meat muffins from a toy oven?  Gack!  Nevertheless, if your vegetarian toddler eats a muffin from these things, they'll get sick and vomit.  Chalk one up for "weird Dev decisions".

Toddlers ...

So the first thing that got me was that Children and Toddlers can't interact anymore.  Not at all  The Child can't get the Toddler a bottle from the fridge, can't play with their toys with them, can't make faces at them, tickle them, chat with them.  It's almost like they live in separate but parallel realities.  It was really nice in Sims 2 when you had a lonely Child who could then talk to the toddler, but no, that's not possible anymore.

I was also a little baffled by the learning that Toddlers do.  They seem to get benefits from, say, the pegbox or the xylophone, but those don't show up anywhere.  Parents can read to them, too, and the game will eventually say they've "learned all they can" from a book, but again, no sign of what they learned anywhere.  This does eventually show up, though -- if a teen who gained some xylophone education picks up a guitar later their guitar skill will start at at least one, and maybe up to three.  So far I've figured out that xylophone does guitar skill, and that the pegbox does logic skill.  There are various kiddy art books they can be read which gains them painting skill.  

While toddlers can play with the block table, it doesn't seem to do them any good, however Children gain mechanical skill with it.  I've yet to find a way to get an early head start on Charisma, which I always liked to do for my Sims while they were kids and it was still fun.

Classes

I had a hard time figuring out where to send my Sims for their various classes.  They like to learn in them and, hey, it's quick.  But It took me forever to figure out that Sims learn to write at the Corporate HQ (the Sims devs obviously haven't been reading the same corporate correspondence I have).  I made myself a list of places to get classes:

• Corporate HQ: Writing class
• Sports Stadium: Athletic class
• Any restaurant: Cooking class
• Military Base: Handiness class
• Research Facility/Science Center: Logic class
• Theatre: Guitar class
• City Hall: Charisma class
• Grocery: Fishing class
• Community School: Painting class

Mom's Diet Influences Baby's Gender

That was a weird one -- and I'd never have guessed but I read it in the forums and it's always worked for me so far (though they say it's not 100%).  Eating apples makes the gender tend toward male and eating watermelons trends it toward female.  Apparently at least three servings should be consumed by mom during the pregnancy.  It's also good to know that these can be eaten as parts of other food (cobbler, pancakes, &c.), or just raw from mom's inventory.

Move That Mailbox and Style That Trashcan

Holy cow the lot mailbox and trashcan are finally movable with the hand tool -- and styleable with the recolor tool! It's so nice to be able to put them out of sight, or make them into less of an eyesore altogether!

Rubber Ducky, Your'e the One!

And they make bathtime lots of fun, too!  Your Sims can buy bubblebath and rubber duckies at the local grocery.  Once home, just move them onto the edge of the bathtub.  The ducky makes your Sim happier with their baths, and the bubbles enable the "take bubblebath" option on the tub.  Mmmm, scrubbing bubbles!