I know in other parts of the world, places like McDonald's will have menu specials and items you can buy exclusively at their place you won't find anywhere else in the world. Is that the case for Vegas? I have never been inside one to know.
I don't know if this is still the case but McD's did have Las Vegas mugs you could buy. They are part of their collectible mug set. Each state has one supposedly.
Nothing as far as the menu is concerned for McDonald's. The McDonald's design/architecture is interesting. The McDonald's on the corner of Sahara and Las Vegas Blvd looks like an old school 50's style McDonald's. The McDonald's next to Planet Hollywood is two floors.
From what I remember, there wasn't anything too notable about the chain restaurants, other than the architecture like @tox said. Most of the hotels have some of the chains in their food court area too.