I would classify a dirty platinum as one that is extremely easy or a kid's game, probably with at least 70% of people earning the platinum. These are generally the games you wished you never played, with no sense of achievement completing it.
For example, I would say Hannah Montana is definitely a dirty plat, along with games such as "Cloudy with a Chance of Meatballs" and "Megamind".
I only have Terminator Salvation, and to be honest it wasn't anywhere near as satisfying as any of my other games as it took about 4-5 hours over 2 days. My only salvation
So yer, feel free to post your thoughts on "dirty" plats below, as well as answering the poll
EDIT: hacked platinums are even dirtier!




















