Wynn and Encore don't have a theme, but I don't think that makes them not unique. They really are both beautiful properties. Bellagio I guess is supposed to have some sort of theme, but it's not all in your face. I don't think a property has to have an obvious theme to make it enjoyable. The only themes I really liked were Treasure Island and NYNY. And maybe Paris.
All of those themed places were put in when Vegas was trying to be more family friendly. So the parents got to tell little Tommy and Susie that they would have to sit in the room all day while mommy and daddy gambled away their college funds, but they get to stay in a CASTLE and won't that be fun. The de theming started when casino execs realized that they no longer wanted to be so family friendly.
Most of them I won't miss.
But I don't get why so many think it's necessary to have a theme or some sort of entertainment for the kiddies to make a hotel/casino different from the rest.