Better speeds, stronger signal and less problems would be my main reasons for wanting to change.
I have 2 of these, they are amazing, only issues is that there is a DHCP bug in the latest firmware(s) if you do reservations etc. Netgear can be slow to fix some things, but they are releasing new firmware every few months.
I prefer the ASUS in terms of features and firmware except they use a really crappy chipset for their DSL side of things, but they have recently released a new model which supposedly uses the same chipset as the Netgear D7000 but it is only in Germany at the moment, hopefully they will release a UK version soon, an ASUS DSL router with a Broadcom chipset would be very cool, even better if they had UK compatible firmware
Anyway, there is no perfect router, but in terms of DSL stability, the Netgear D7000 has probably the best chipset and modem firmware at the moment.