Seventy-five percent of the cards I get are of the family picture or kids' picture variety. I actually enjoy seeing how big the kids are getting, especially from people I don't see or talk to much anymore.
I don't seem to get the holiday letters, but I know of them. If anyone cares to share them here for sport, I'd read!!
Now to answer the question posed: My least favorite are the ones that people get printed up - with their names already printed, so they don't have to take the two minutes to sign their names. They even get the envelopes printed (not a mail-merged sticky printed out on the computer, which I have done before) with the complete "to" and "from" addresses.
I don't send cards anymore because it was getting really expensive (this was before Facebook). I was sending over 180 cards out (about 75% family). With the postage now sitting at 44 cents ($79.20), and cards coming maybe 20 to a pack at $10 each ($90) - it's now lot of money. I feel bad, but then again, I rarely receive more than 20-30 cards a year, despite sending so many out. I know it's not about that, but that extra expense, and the time writing out every card and envelope (I always used a red or green felt tip pen and wrote in really nice cursive) takes its toll on my carpal tunnel syndrome. It usually took a good three weeks to get all my cards done (no, hubby didn't help, lol, he has interesting handwriting).
It's too bad, because it was a lot of fun. Maybe if the economy gets better, I'll return to the practice in a year or two, because I enjoy the process. With Facebook, however, I can't imagine how many more cards I would be sending out.