Maybe not "hidden", but none of the others are hidden either, but certainly obnoxious are taxes.
On my upcoming itins:
Traveler Base fare Taxes & fees* Total
Traveler 1 GBP 254.00 GBP 252.30 GBP 506.30
Traveler 2 GBP 254.00 GBP 252.30 GBP 506.30
Traveler 1 GBP 557.00 GBP 238.50 GBP 795.50
Traveler 1 GBP 557.00 GBP 238.50 GBP 795.50
Traveler 1 USD 224.00 USD 58.80 USD 282.80
Traveler 2 USD 224.00 USD 58.80 USD 282.80
GBP 1846 in airfares and 1040 in taxes. 36% tax. It sucks flying ex UK.
Also, the LCC's in Europe will also charge you to pay by credit card - per person, per segment or ~3-5%.