The Mt. Gox FAQ reads:
Unfortunately we do not accept any credit or debit payment
Why is this so? It seems like the most natural way to buy bitcoin would be to pay for it using money in my bank account. However this very method seems to be avoided almost everywhere.
Main reasons :
1) Bitcoin is anonymous, free and irreversible. Credit and debit card transactions have to be reversible, Paypal transactions are reversible. So such a system would be a gold mine for card thieves, money launderers, scamers, etc. A relatively small website cannot face the risks.
2) If you're dealing with credit cards you have to follow some regulations, pay charges to the card emitter network, etc.
No one will trade you Bitcoins for credit/debit because these kinds of transactions are highly prone to fraud and are easily reversible, while the Bitcoin side of the transaction is not. It's entirely too easy for me to call my bank and have a charge reversed - and now I've got your coins and my dollars and you have nothing.
"money from my bank account" would be more like Dwolla or Popmoney. There are plenty of exchanges that accept Dwolla.
Debit cards, and more so credit cards, are not bank to bank transfers. The money goes through a third party processing network which will have its own fees and chargeback requirements and rules, all of which get very complicated. I'm sure you've heard of some of these networks, and seen the logos for many more on ATMs:
