This question was inspired by this link, http://www.jazclass.aust.com/bridge/br14.htm which required a five card suit with two honors, at least one of them a queen or higher, for a one-level overcall. Hence, QTxxx was acceptable, but not Qxxxx or JTxxx.
A variation of this rule is that you need five cards in a suit (headed by two honors) for a one level overcall, and six cards in a suit (headed by two honors) for a two level overcall.
Another rule I've heard is that the number of honors plus the number of cards in a suit ought to at least equal the number of tricks. Marty Bergen, at least, in "Points, Schmoints" would allow a one level overcall (seven tricks) with four cards in a suit if there are three honors. Likewise, he and his partner, Larry Cohen, would allow a two level overcall (eight tricks) with a five card suit if it were headed by three honors. They would, of course, want two honors (however defined), for one and two level overcalls with five and six cards in the suit respectively.
Are there any rules that most experts agree on regarding overcalls?