I guess my definition of gimmick is different than yours.
"An innovative stratagem or scheme employed especially to promote a project"
Staples of video games for decades (running, jumping, climbing, killing) are not gimmicks in my opinion, just part of the design. Being an assassin is not a gimmick as that is one of the four elements of successful commercial game design (pirates, aliens, robots, ninjas (okay, an assassin isn't exactly a ninja...)). Polish, is not a gimmick, just a good feature. That is where this game really does shine. The cities are wonderfully realized. They look like cities, not tiny villages that people are calling cities.
Like I said earlier, the only gimmick I see is the blend into crowds feature. It is a gimmick. It doesn't always work, but it is often funny (both when it works and when it doesn't).