The deal about a credit not being equivalent to making a minimum payment is lame, but I'm not that surprised. They'll do anything to charge a fee these days. For instance, your payment must be received by noon or 1pm on the due date (for some issuers) or else it's considered late. All kinds of screwy stuff like that. Another thing you discovered in this is that you shouldn't take any of these fees sitting down. If charged such a fee, call and complain about it. You won't get anywhere if you are repeatedly doing something wrong (like paying late), but an occasional fee should be waived if you are otherwise in good standing (again, varies by isssuer).

The deal with the promo interest rates is very standard. Payments are always credited to balances with the lowest interest rate first. So yes, you would get screwed if you transfer some balance to take advantage of the interest rate, then go out and make a purchase (assuming the purchase interest rate is higher, which it normally is). You're getting a free ride. But enough other people make this sort of mistake that it's still worth it to the card issuers to offer these teaser rates. Also the terms always state that they can raise the rate for late payment, etc, another thing that can trip some people up. The (sad?) truth is you are getting a free ride at the expense of those who are not as smart with their credit.