Well all very interesting people. May I say that asking the crew for a upgrade is a sure fire way NOT to get one however.
But you are all quite right in saying that you can be flagged as suitable for upgrade. But this does not gurantee you one.
May I say that a friend of mine often travelled Europe an claims to have been upgraded 50% of the time. His recipie for success was to flirt with the check in girl and when asked on where he'd like to sit said "in club!!" with a cheeky smile. Believe me even though you may have had **** day - taking it out on the check in staff and throwing your toys out of the pram is not going to get you that isle/window/bulkhead seat or upgrade. Be nice to them - it may get you a more comfortable ride home.