I like the sort of design that Roxy Palace uses – other casinos use it, and I find them more intriguing that casinos designed strictly for aesthetics because they have more visual interest.
I especially love the Roxy Palace slots skin http://www.roxypalace.com/?skin=2&btag=rpaff846
(pardon the aff tag)
I like blues, and I feel that blue is the best color to use for casinos – research shows that blue is the colour that people associate with integrity and honesty.