When the 60D came out it was a bit of a surprise that the XXD class was downshifted into the 'super Rebel', meaning it had a smaller, polycarbonate body, lower framerate than its predecessor, and no AF microadjustment. At the time, these downgrades were necessary in order to differentiate it from the 7D, which came out in late 2009. But since then, the advantages the 60D had over the Rebel series like 5fp and the 9 cross-point AF system have diminished ever since the T4i was launched and matched these specs. Today, the rebels even have an advantage over the 60D as they feature continuous autofocus in video mode. The competition hasn't stayed quiet in the last 3 years either, with Nikon's D7100 and Sony's mid-range SLT line giving generous performance specs.
Likewise, the introduction of the 1DX and its combination of a full-frame sensor and high framerate has left a bit of breathing room for a successor to the 7D to upgrade its framerate, AF system, and weather-sealing. This means the XXD class will once again be able to return to its former 'pro-sumer' market position, like the 50D was in.
So what are the biggest improvements in the 70D? At this point, it looks like the sensor, framerate, and AF systems are the major additions, with the 70D's autofocus system trickling down from the 7D. This finally marks the end of the 9-point AF system in the XXD line, which has been used by Canon for the past 9 years, with only one revision during that period. Also included is built-in WiFi, like on the 6D, though there's no mention of GPS. If GPS was indeed omitted, it may be in order to return to an all-magnesium alloy outer shell - the 6D had a polycarb top plate to allow the GPS to send/receive signals. Externally, there isn't much else we can determine except the size.
Click for full-size image sources: canonrumours, canon.com
As you can see above, the sizes are pretty close when using the lens' 58mm thread as a size reference (thankfully Canon likes to take their product shots at identical angles). Aside from the 70D having a slightly higher 'shoulder' on the side with the mode dial, they're pretty much identical when it comes to main body dimensions. Any other differences can probably be chalked-up to the 18-55 IS STM being slightly longer than the 18-55 IS on the 60D, which will slightly fudge the scale. Sadly this means the series won't be returning to the full size of the 50D, though the 60D wasn't exactly small. By keeping with the 60D's dimensions, and with a vari-angle LCD confirmed, there's little chance of the joystick controller making a return. Control-wise, the 70D will probably be identical to its predecessor.
The viewfinder prism/flash hump does seem larger on the 70D though, indicating that there may be a larger viewfinder, possibly the 1.0x one seen on the 70D. Since it inherits the 7D's AF system, that's a pretty good possibility.
Obviously we won't know until the announcement is the made shortly, but until then this'll have to do.