Why do some transactions take minutes and others take hours? I sent some bitcoins to an address and then looked on blockexplorer and it was there in about 10 minutes, which is what one should expect.

I did another send to a previously unused address and it took hours for the network to acknowledge it.  

I contacted the help desk at Mt Gox and they said this happens fairly often.  My question:  Is this a known phenomena and if so what causes it?
Four factors affect this:

1) The size of the transaction. Smaller transactions get a lower priority. Abnormally small transactions look like spam.

2) The transaction fee. Abnormally low transaction fees may mean the transaction isn't even relayed. Miners have an incentive to include transactions with higher fees.

3) Luck. Some miners don't include any transactions. Some don't include any transactions without fees, even if the fee would normally be zero. So it comes down to who mines the next block or blocks. Also, 10 minutes is just the average. You could create a transaction and then by sheer bad luck the next block isn't mined for 20 minutes.

4) Security. The client 'dribbles' out its own transactions rather than broadcasting them to try to conceal the origin.