I think snowy / Christmas themes or just temporary edits to themes look ok as long as there is too much of it, like too much snow would annoy me if it was all over the text im trying to read. I do like to keep my website in the festive spirit lol
I did add falling snow to my current forum. I don't really do much blogging anymore so I haven't added snow to my blog. I like the effect of snow falling in the background behind the content though as it looks wintery and snow is beautiful.
We never even talked about adding the falling snow effect here on the forum last year for the Christmas holiday month. I like the effect though depending on which site that it's implemented on. I've seen some forums where it looked great and others where it looked terrible. I think the reason why it looked bad on some forums was due to the layouts themselves. That makes a big difference.