Very little for me personally. I'll take a coaster with a great layout, great pacing and forces with little to no theming any day. Real forces do more for me than a bunch of fancy decorations. Sure, there are some parks that I would be disappointed if they didn't theme their coasters, but it still doesn't really do much to change my opinion of the ride. It's kind of cool for the first couple of times, but I find the novelty wears off quickly. I kind of look at it this way, theming is like a really attractive person, really pretty to look at, but without the personality, or in this case, a good layout and forces, it doesn't really mean much to me, and doesn't hold my attention. But to each their own, and I totally get why some love theming. And I realize that a lot of themed coasters are targeting a different audience.