What are the “high performance characteristics” of a Boeing 777?

News of the potential further stretch of the A350-1000 have seen pretty much all journalists discuss the "high performance characteristics" of the competing Boeing 777-9 with Middle Eastern airlines. For example:

[...] the new Airbus jet would target airlines that need high-capacity, long-range aircraft but not necessarily require the same high-performance characteristics that have made the 777-9 so popular with Middle Eastern airlines. (The Gulf region can be hard on an airliner.)

What exactly are these? Why is the 777-9 suited to them and the A350 not?

• I agree in principle with the take-off performance consideration. I would propose to refine the second part of the answer ( higher thrust from the engines and larger wing area) -> higher $T/W$ (maybe(probably), we don't know Airbus values yet). Another aspect that influences take-off performance is $L \over D$ of the flapped configuration and $Cl_{max}$ – GHB Mar 9 '16 at 23:51