Thinking you need to use manual mode all the time
If your subject matter sucks, it sucks regardless of how much effort you put into shooting in manual mode. Sorry.
Auto is smarter than you are. It knows what kind of aperture/iso/shutter speed you need to take the picture. Don’t worry about it. Point your camera at something interesting and press the button, even if you’re in “dreaded” AUTO. That includes shutter priority and aperture priority, which I personally use all the time because my camera is smarter than I am.
Thinking you need to buy (even) more gear
This series of shots has quickly risen among my personal favorites. I shot these on film with a <$100 camera while riding a commercial airliner as a passenger. The camera is from the 1980s, it was marketed to consumers and not pros, and the lens cap says 1984 Olympic Games (which is badass in and of itself.) Taking these pictures for far less than the price of most insta-crap cell phones says it all.
Thinking you need the best and newest lenses
You need fast lenses that can help create interesting pictures, not expensive ones. You need lenses that are capable of isolating your subject against the background, that are also versatile enough to shoot with everything in focus. You need lenses like Canon’s $120 50mm f1.8 and Nikon’s $120 50mm f1.8 lens. You can take all kinds of pictures with these lenses without breaking the bank.
Thinking you need to shoot RAW
RAW is fine. It’s just not all that much better than JPEG, which takes up a fraction of the memory space and provides more than enough data to digitally dodge/highlight your photo to any reasonably amount. RAW does not make a better picture. Better pictures make better pictures.
Thinking you need to edit the crap out of everything
Seriously, please don’t. HDR is the visual equivalent of nails on a chalkboard.
Thinking one battery and one memory card is enough
It’s just not. Enough said.
Thinking a nice camera will get you good pictures
The camera really, truly doesn’t matter nearly as much as the effort you put in. A picture in broad daylight with harsh shadows and ugly colors is always going to be worse than a beautiful sunrise shot of the same scene. A better camera helps you take higher quality images of the same things you’ve always been shooting. Your attitude and work ethic – putting in the time and effort – helps you take better pictures regardless of what kind of camera you have.