Not sure how to capture "soft" stuff like health care and/or vacation etc. I recently was surprised to learn the in New York an employer doesn't have to pay out unused vacation where as in other states they have to by law. This sort of stuff can certainly catch someone off guard when leaving a job to relocate.
Otherwise it's a very cool concept...
Sales taxes and property taxes figure into prices of goods. The site already says to include sales taxes, though it would help to have rates noted explicitly. Property taxes should appear as an expense under housing.
I'm in NY, my property taxes are about 60% of my mortgage payment.
In small cities like mine, it's killing the housing stock, as taxes are increasing faster than wages, and people are deferring maintenance.
numbeo.com has that (but Expatistan's display of price in both currencies is nice)
I would prefer this one focused on doing what it does right.