Having recently stepped up from a Fujifilm Finepix S1800 to a Canon Rebel SL1, I have to say that the tool does have a lot to contribute to a good picture. With my Fuji, anything over ISO 100 had noise levels that even a rookie could spot. Without ISO flexibility, one third of the triangle is gone and with it one third of my available options. My Fuji simply didn't handle well in low light. With the Canon, I can crank the ISO all the way up without losing nearly as much in the noise department. My Fuji's ISO 200 noise level looks a lot like my ISO 6400 noise level in the Canon.
That said, a lot of the sensor debate involves a single question: What is the end goal? If you're using the camera to capture moments of the nose miners for Gam-Gam in Coral Gables, my Fuji is enough camera and enough sensor. Set it to auto and let 'er rip. If you have any ambition to take creative control over your output and want to possibly have print quality results, the Fuji is not going to help. I had minimal control over depth of field, and any of the artistic shots I want are simply incompatible with that platform. The car that will serve well as a work commute car may not handle well as a drag race platform and I feel cameras are much the same. If you're just documenting nose miners and nothing more, using an ILC with 37.5MP is like using a sledgehammer to kill mosquitoes.
Truly, I'm not really needing more than what I have right now, and any picture I post online is going to end up being cropped way down because of size and display restraints. Thus, my 18MP is overkill if that's all I ever do with it. Still, I do intend to get into stock photography eventually and having more camera than I'll need will be much better than not having enough.
End application is really the question for me. If the user has plans to email the pictures to a relative living away, 18MP is only going to increase bandwidth loads for small gains. If the user has plans to create and sell prints, I'm not sure 18 is "good enough" for that.
Overread has a key point: stepping up does make you more aware of options you may have overlooked. I dove into the Fuji pretty hard and got some good shots but when I got a "better" camera I actually got serious about it. At the same time, a lousy mentality about photography is a lousy mentality. I felt I was "better" than the camera I had whereas now I know the camera I have is much more capable than I. In many things, that's important. I didn't always feel like I needed to bring my A-Game to the table with the Fuji. With the Canon, I know I can't be lackadaisical about my part of the puzzle. I actually have enough control to make a good picture or a bad one and if the shot doesn't come out I can't really say the camera is the weak link. With my fuji, I had the "ISO is so noisy" line to fall back on for a lot of things. That's why my Fuji isn't for sale; it's not just that it has a really low resale value but that I've given it cubic miles of emotional baggage with my frequent criticism.
