no player rentals or temporary trading. it just makes for stupid fantasy football activity. say a guy with two good QB's who's missing an RB for the week 'rents out' a QB for an RB for one week. this goes against fair play and violates the fair competition that makes fantasy football fun.
how about a rule that there are NO give-backs. once you trade a player to someone, you can't have him on your roster the rest of the year. it prevents the trade-back silliness, and also prevents 3-way trades to get around these restrictions.
just my thoughts. player-rental doesn't make for a fair league. and it also leads to guys giving their 'friends' players in unfair trades when they're having a down-year.