I don't get the Deck idea of the month. Isn't is suposed to show a deck idea created this month?
No...
What is your method for determining the limits on the X of the month?
For the card idea I include all cards I notice and are at least in the polls that month. On slower months I also include all cards in the crucible.
My usual method is to give the spotlight to any hot, popular decks that became a community favorite during the past month or so, that way it's relevant to the community's interest when reading the article. However, with a lack of game updates this becomes very hard to accomplish to the point where I'm usually left with selecting a deck from the Community Recommended Decks board and discussing it. So really my only limitation is to use a deck that's popular with the community.