Usually about 2/3 to 3/4 of pax ask for an upgrade (the rest have already paid for it!).
In the olden days, at T4 Check-In, when smoking seats were still available, I was checking in a JFK flight in two column queues. First I get a lady who tells me her company had arranged an upgrade, and after a few minutes of her arguing I asked her if she had her 'upgrade voucher' (which didn't actually exist) and she then says 'oh that little voucher thing, I left it in the office!'. Bring it next time madam...
The second was a man who wanted a smoking window seat. He checked in late. There was only a n/s window or smoking aisle seat left. He stood and refused to move until he got his seat. I offered him the n/s aisle seat with the option to go to the smoking section for a smoke. He demands an upgrade. Eventually I tell him "take the non-smoking window seat, and if you need a cigarette, I'm sure no-one will mind if you have a sneaky one out of the window" at which he agreed, said "it will have to do or you'll make me miss my flight", took his boarding pass and went.
Brain in suitcase (check), suitcase checked in (check).
(sorry cabin crew xx)