What's wrong is that the total cost isn't being disclosed up front. It's just like advertising "$10 iPod" and then charging $200 in shipping and handling.
What's really irritating (to me) is passengers who are using advanced check-in so they can go directly to the gate -- with their oversize bag in tow. Once there, they are told they will have to gate check the bag but most airlines don't seem to be charging for gate checked bags. I don't care about the airlines losing the revenue, but I do care about the hold-ups and hassles it causes during boarding.
I wonder how long it'll be before they start charging "boarding pass handling fees", "seat cleaning fees", "oxygen fees", etc.
GBP 40 for turning up to a Ryanair flight without a boarding pass: "http://www.guardian.co.uk/money/2009/may/14/ryanair-online-c...
And it's GBP 5 to check-in online at home: http://www.ryanair.com/en/questions/table-of-fees
If memory serves, the penalty was either 7 euro or 12 euro per kilogram.