ainwood is saying that we need to start with a "vision statement" that describes the destination we seek (or the state we want to achieve). If there is not consensus on that, there is no point in debating the mechanics of achieving various ends.
So here is a shot: We want an XOTM system that:
1. Encourages participation of the elite players.
2. Encourages participation of moderate level players
3. Encourages participation of beginners.
4. Allows comparability of games
5. Optimizes learning for all
6. Is fun for everyone who participates.
7. Does not overwhelm the staff (

)
8. (add your own)
Some of these goals are potentially in opposition: what encourages elite may scare off beginners. If the answer to that is tiers of games, then comparability suffers the more the tiers are different.
It may come down to finding consensus on goals,
then a consensus on their relative importance (weighting the goals). Maybe comparability is less important than participation?
Finally, a particular game may focus on some goals and not others, and the next game may reverse this. Might be the only way to have "something for everyone" over the system, if not in the individual games.
dV