There's no surprise that SIN has consistently taken out the award, year in, year out. I believe it is one of the few, if not maybe only, airport in the world which has been designed for humans to enjoy being at the airport.
Even if a bit kitsch in places, the aesthetics of the airport speaks to creating an enjoyable rather than utilitarian environment. Clear signage, accessible, effective movement between even the farthest gates, plenty of shops, food retail and other facilities, often at fairly reasonable prices, friendly and curteous customer service. Plus of course effective transport to and from the city centre.
I don't consider airline lounges in any of these assessments. I know Skytrax has separate awards for lounges, but I don't know if the survey feedback about lounges affects the airport rankings.
Probably the biggest bugbears about SIN are security at the gate for T1, T2 and T3 (some might argue this is an advantage); and the disconnectedness and transfers to/from T4.
Anyway, DOH beats them out this year... or the Qatari government has dropped a lot of money into Skytrax. Admittedly DOH does look quite elegant and sophisticated, but I'm not certain people would want to spend lots of time there. Then again, what would I know - my only experiences in DOH have been 95% in the lounges.