I have no issue at all with return address labels. It's not standard to have the sender's address on the envelope at all, here (England) but I know it is elsewhere and I have labels I will use on anything going overseas. If I'm sending a card to someone in this country I will only put a return address label on if I think they may not have my address or if I'm uncertain of theirs (at least that way, if I have their address wrong the cad may bounce back and I'll *know* I have it wrong!)
It would be nice if you gave me some clue as to who the card is from, though. I have recieved a card from (I'm pretty sure) the same person for the past three years. It has nothing inside other than a signature which is illegible. I'm pretty sure it starts with either an 'N' or and 'M'. There's nothing else in the card, no return address. I have absolutely no clue who it is from.
For me personally, it's easier to handwrite the mailing address than to try to work out how to print lots f different addresses but if you are organised enough to print labels I don't care in the slightest. (FWIW, I would also not care if the card/letter was a Wedding invitation - of course if you have the skills to address your invitations with beautiful calligraphy I shall be very impressed, but to me, the reason to have the address on the outside of the envelope is so the Post Office knows where to deliver the letter - having typed or mass-produced stuff **inside** is different)
In terms of what goes in the card, that depends entirely on my relationship with you.
People like my parents and brother, who I shall be spending christmas with, I will probably just write 'love from M', and expect the same from them. People I'm close to but am not going to see I'll add a bit more - but not a lot, as they tend to be people who I speak/write to fairly frequently so there isn't much catching up needed.
others may get anything from just their names, and my signature to a full letter, depending on our normal interactions.
I haven't ever sent a 'round robin' letter, I don't mind getting them (I tend to start with the assumption that if you are sending me a card it's a gesture of good will and an expression of intent for us to continue to be friends - I'm not going to worry to much about exactly what's in it) but that said, I find most round robin letters are either full of 'news' I already know, because I'm in close enough contact that I knew your niece got married (she is my sister, after all!) or that you had a lovely week at the seaside (you sent me a post card) or else we are sufficiently distant that the information isn't very interesting. If you're sending a round robin letter I do think it is best if you at least personalise it by handwriting my name at the top, and perhaps having a one line, personalised comment at the end, because it feels so impersonal otherwise, but mostly I take the will for the deed and assume you mean well.