Here's what's important to us...
We are modeling with our toddler to say please and thank you. We coach him when he asks us for something, but are not "what's the magic word" kind of people. This is a habit that children take a long time to learn, and denying them what they are needing doesn't help. But we do use it when we ask him for his cooperation in a task. (Tone of voice when asking for something is a bit more important... no one likes to have orders shouted at them.:))
RSVP- You know, as someone who plans gatherings, it's great to hear from people regardless of if they can make it. I'm sure moms planning parties feel the same way. To me, it's standard courtesy. If you care enough to come, RSVP. If you care enough to be invited again, RSVP if you can't. RSVP is French --Repondez-vous, s'il vous plait--- which translates to Please get back to us in a timely manner whether you are coming or not.
Thank You's- as the auntie of two sets of nephews, I have certain feelings about this. When I take the time to send a package/present, I like some sort of acknowledgement. One of my nephews (and his mom) don't "do" thank you's, and I'm always a bit less excited about gifting them than I am my other set of nephews.Their "thank yous" are often a card that has pictures and stickers from the kids to us, which is age-appropriate for little ones. A note is nice too, and teaches children how to compose a thoughtful correspondence to another person, which can be a lost art in the days of email and texting. Thank you cards are good for two reasons: the giver's genorosity and effort is brought to the attention of the child (so they don't take it for granted) and it's a lifelong skill that is very useful in maintaining adult professional relationships later on. As adults, sending a thank you note to a friend for their help, or another professional for their advice, insight or time is a great way to cement relationships. (I could go on and on in regard to this one!)
Opening doors: First person to the door holds it open for others, regardless of gender. People passing through should return the courtesy and be sure to say "thank you". The other day my husband held a door open for a woman and her younger son, who ignored him completely. "That's a great way to teach your son how to be an entitled jerk" he muttered as soon as we were out of earshot. Which leads me to...
Teaching our kids: it's not what we teach our kids to do, it's what we do ourselves that our kids will learn from. Saying "excuse me" when cutting in front of someone at the store, saying "thank you" for small kindnesses and courtesies, these are important. Saying "please" to servers and other people we interact with. Holding doors open for people with their hands full or pushing strollers, or giving someone on the bus a seat because they need it more than we do...children know kindness and respect when they see it.
You didn't mention table manners, but those are important too. Elbows on the table aren't the biggest deal in our house, but sitting down while one is eating, chewing with one's mouth closed, and not speaking with a mouth full of food are good habits. Being a pleasant dinner mate has it's priviledges later in life.
For what it's worth, I'm 38 and hated writing thank you notes... and I'm so glad my mother made me do it!