Well, to someone who flies often it is obvious that upgrades are dependent on which flight. Or to someone who stays in lots of hotels.

If you want a good shot at an upgrade take some random US Airways flight with AA status. But if you are flying Delta out of Atlanta, just give up before you even start.

The issue with a lot of these companies is that upgrading isn't effortless. Because sometimes you aren't even put on the upgrade list unless you specifically request it (like with the AA/US merger, and code-share flights). With Marriott you have to ask for an upgrade-they don't like to hand them out, apparently.

This makes people unhappy, because they have to conscious remember to ask for an upgrade, and then get rejected.