The 50D added features, compared to the 40D, while the 60D subtracted some things, while adding others, so the 60D was more of a step to one side, along a different path, than a step forward. The 7D was a true leap forward, the logical true successor to the 40D and 50D, in my opinion. The 7D is still current, though more than four years after introduction, many wonder what its successor will be, and when it will happen. The 7D has ergonomics and controls that are quite good for my hands and brain.
The 70D, continuing with the maddenly-different 60D consumer-ish controls, has not interested me, except to indicate some AF technology that might be present in a possible Mark II version of the 7D. The 70D is certainly a step forward for those with a significant interest in video. (I am not interested in using DSLRs for video.)
The 6D certainly merits consideration, but make sure its auto-focus system suits your needs. The 6D's central AF point is very sensitive in low light, but its other AF points make it a lesser camera for some users, especially action shooters. I have been tempted by the 6D, but held back due to its ergonomics and controls. I am hoping some of its low-light AF technology might be incorporated into future 7D and/or 5D model.
I use a pair of 7D cameras at work, which is evidentiary/crime scene photography, at night, as part of my police patrol job, and am largely satisfied. For me, the 7D is the successor to the 40D, and I have no immediate need for an upgrade. I have kept up-to-date with Speedlites; good lighting keeps "old" cameras quite relevant. My old 40D still does very well with the help of good lighting!
Notably, the 7D's 2.0 firmware update added significantly to this camera's capability, making all 7D cameras a "virtual" Mark II, in some ways. A Canon executive, during an interview, indicated there would be a successor to the 7D, but there has been much debate over exactly what he meant.
With no successor to the Nikon D300s, Canon can, presumably, take its time updating the 7D line.
If no 7D successor becomes apparent by mid-2014, I may buy a third 7D, so I can always have two 7D bodies with me, while one may be away at Canon being serviced.