Welcome to FT, granada77! The issue is not tax (usually). The main issues are fuel surcharges (called YQ if you ever see a detailed breakdown of "taxes").

US based airlines usually don't charge a fuel surcharge when flying with them or another Star Alliance Partner on an award. Others (like LH, BA, QF) will charge very high fuel surcharges on all bookings, no matter whether it is an award or not. LH, QF etc will charge the LH etc fuel surcharge even on segments operated by airlines which have no or a much lower surcharge.

For example, when you book a RTW ticket with QF, it can cost you around $1500 in taxes and fees. The same ticket, flying the same routes, airlines etc issued by AA will cost $1000 less.

Also, stopovers in London can be very expensive due to the APD. This is another source of high taxes.