If you consider the quality of your photographs at present and your aspirations to improve, what is the most critical shortcoming standing in your way? Look at your favorite photographs and ask yourself what made them special. Do the same for photos taken by others.
Time - are you spending enough time, taking enough photos, experiencing enough situations that you are improving at the pace you desire? Do you review your photos to learn how you could have better composed, focused or exposed them? Do you seek input from more experienced photographers, or just family and friends?
Body - are you using your current body to its full capability with regard to your needs? Is there any one thing which your body lacks which would improve your photos? Think about it's frame size, noise, resolution, FPS, size, weight, etc.
Lenses - do you typically have the lens you require for any given situation? Long enough, wide enough, fast enough? Would a different lens really make that much difference?
Post-processing - are you happy with your software, hardware and skill level? What would help you improve? More time spent learning processing or better kit?
I have observed a whole of people spend too little time learning the craft before jumping ahead to the latest and greatest equipment. I guess this is human nature.
I had a Rebel series and was very happy with it up until I started taking astrophotos, then noise immediately became an issue. This was the primary reason for a full frame upgrade. I've been very happy with the reduced noise, but less happy with huge file size and tremendous camera bulk and weight. The 5D2 is not particularly suited for travel, IMO, as it not inconspicuous and gets pretty heavy at the end of a day. Secondary reason for the upgrade was that I do quite a few bird photos and was cropping pretty heavily and thus desired improved resolution. The 5D2 is great for this, but lens length and bird stalking ability are far more important. I currently live in a dangerous city where carrying a 5D2 around is an invitation to get mugged, so I mostly just use the iPhone, which has higher resolution than my first digital camera and more on-board processing than my last laptop. Photo quality, particularly with Instagram, is appalling, but nobody seems to care since the content/message is far more important for stuff posted to Facebook.
What about you?