I have some mixed feelings about this...
First: I send out a letter myself, however I try to keep it as humble as possible. I don't want it looked at as a brag sheet more as info that I don't have the ability to share with everyone I send a card to, but maybe are interested in the info included. Am I babbling?? The first year we sent one out (3 christmas's ago) we wanted to make everyone laugh...so we included a picture of an animated christmas tree that was larger than half a sheet of paper, and "bragged" about some funny things that had happened to us that year. (our dogs getting skunked, etc). The year after that we sent out a similar letter, with a similar picture taking up about half of the sheet, and the vast majority of the letter was about our son (who was born that year), with a brief sentence about our dogs, one for my husband, and one about me. This year we left the picture out, and mentioned the major milestones in our son's life, along with fun activities the family did. Again we include only one sentence for my husband, and one for me. It's not our intention for this letter to be a "brag sheet" only to share things that we might not be able to share every day but are important to us anyway.
Second: I get some of the type of Christmas letters where people are using them as a soapbox for their lifestyles, or as a brag sheet. There's at least one that we get every year that my husband and I laugh about cause it's all about one person and all the great things that happen to that one person and leaves out all the info about everything involving only their spouse. It's so lopsided it's impossible to miss, and amusing to read.
Third: I look forward to reading the Christmas letters more than I do in reading the Christmas cards. It seems VERY few people take the time to make a Christmas card personal and the letters are a way to feel personal even if they aren't. I do get annoyed with the soapbox's and the ones filled with a person's need to feel important.
I feel offended at the people that say they see them as only brag sheets...people take a lot of time putting their lives down on paper and sometimes it can feel a little odd to share personal parts of your life...see what it looks like on paper and feel the need to fill it with more interesting things. Understand that's their insecurity and not the need to compete with another. I personally try to be humble when writing mine...but maybe just the act of writing a letter like that prevents me from being humble...hmmm...interesting thought!
To addresses your other questions...I like the idea of making a poem and keeping it lighthearted even if you need to address sad or difficult information. The holidays are a time of merriment and cheer, I would imagine most people would want to avoid a christmas letter filled with low moments. You can make it whatever format you'd like...I personally like the news letter format..it's very cute!