All the time, people come up to me and ask which camera they should buy. Usually they try to choose between the latest and flashiest model they can afford, but mostly that's the wrong question to ask, so I ask a counter question: "In what ways does your current camera limit you?"
Barely anyone can give an answer to that simply because very few people will ever reach the limiting factors of their cameras. I still use 10 Megapixel cameras simply because I had 30x40 inch (75x100 cm) prints made with those hanging in galleries and no one ever told me that my camera had too few megapixels. Since I rarely shoot high speed sports, I also don't need more than 3 fps and the few times I do, I deal with it. Now if my main source of income would become high speed sports, I would consider getting a camera that shoots at least 5 fps. The current limiting factors I see in my cameras are ISO, AF points, and image sharpness. I already use high end lenses, so at this point in time, my image quality is not limited by my lenses but by my camera bodies. So far I haven't seen a camera that does all of these to make an upgrade worth it for me. However, that may change in the future and will be a major buying point for me. Since I travel a lot and use gear in all kinds of conditions, of course the body has to be weather sealed and be made of magnesium alloy. Unless all of these things come into place, I won't upgrade my cameras. As you can see, I have evaluated exactly where I have a need and will only ever upgrade once that need is met.
If you are just unhappy with the image quality, you may want to consider buying better glass than a better camera, since that's the limiting factor for most people.
Another question to ask yourself when buying a new camera is: "What kind of photography are you trying to do?"
If you are doing sports photography, you will have different expectations from a camera than if you would do portraiture or if you would do landscapes. I know many sports photographers who use only one AF point during the entire lifetime of their cameras, while I know portrait photographers who even think that 61 AF points are too few. At the same time, most portrait photographers would be perfectly happy with a burst rate of 1 fps, while many sports photographers think that 8 fps is still too slow. If you only shoot indoors, you will also not need a weather sealed body. How much you shoot also impacts what kind of camera you should get. If you shoot a lot you may be better off getting a camera with a higher life expectancy and upgrade to the new model whenever your old camera gives out instead of having to buy the same model again at the end of its life cycle.
Also consider that if you mainly shoot one thing but once in a while you get jobs for other types of photography, it may be cheaper for you to just rent the camera body required for that type of assignment (or borrow it from a friend).
