I find that my SLR pix are more technically sound, carefully and precisely composed, and "serious", and that I often shoot rangefinders more in the style of a point and shoot camera, i.e. not caring so much about the technical aspects and just having fun shooting loosely. I get many more keepers that are exactly how I intended them to be with SLRs, and I have more fun shooting lightheartedly with rangefinders.
As a result of all of this, I am more prone to snap away with a rangefinder, and I burn through film something fierce. With no technical feedback, such as seeing that something is fuzzy or not engaging the edges of the frame just how you might want it to, I am more likely to just take the shot without "editing" or "manipulating" it as much. SLRs lend themselves to a more designed approach to images, at least in my own shooting.
In short, I like using rangefinders because they feel like the ultimate-quality point-and-shoot. I hate using them for precise work. They just don't fit with that sort of approach to me.
All this being said, these are just my natural tendencies when using SLRs and rangefinders. The thing is that I can do anything I can do with a rangefinder with an SLR, but I cannot do anything I can do with an SLR with a rangefinder. It is not about the camera. It is all about attitude. I take rangefinders not just when I want to shoot loosely, but when I want to not be bothered by a bunch of technical details and equipment, and just shoot and see what happens. I can do this with an SLR, but it requires more work (heavier, larger, more precise, etc.) Using a rangefinder streamlines the process of shooting loosely for me.
However, none of this has to do with light, contrast, lenses, etc. It has to do with choosing the right tool for the desired shooting experience and/or result.