The problem with blockchain.info wallet is that although it says it follows the fee policy of the Bitcoin reference client (the default setting in account options), it doesn't. Even though this is set, the send box likely defaults to 0 BTC as a "miners fee" (sic) (a transaction fee).
Default Fee Policy (settings)
Blockchain.info transactions are free of charge. However the bitcoin network may require an additional Miners Fee which affects the speed your transactions are verified and helps support the network. Change the settings below to alter the default Miners Fee Policy.
◎ Frugal - A Miners fee will not be included unless absolutely essential. Transactions will be significantly slower to confirm.
◎ Normal - Follow the fee policy suggested by the mainline bitcoin client. Recommended.
◎ Generous - Fees will be set to ensure transactions are high priority (0.001 BTC Base Fee).
You may notice that when you send a transaction, you are given an option "miner fee", however you are given no idea of the appropriate amount. The base amount is 0.0005 BTC per KB, but no calculation is made showing you the correct amount to pay or what the transaction size in KB will actually be.
Most transactions will be less than 1KB, but if you've received a bunch of payments (such as from a gambling site) and send them at once, the transaction may be much bigger, and paying just a 0.0005 fee could result in "ignore this spam" priority.
Setting the fee to zero will get you hours and hours of your transaction being ignored by miners in any case.
You may note that none of these "my transaction isn't confirming" threads are from users of the Bitcoin reference software.
