The main differences with 7D are that it's designed from the ground up to be a more pro-oreinted camera... capable of action/sports, increased durability and sealing, customizeability. It's sort of a "1D series Lite" and the most pro-oriented APS-C/1.6X DSLR Canon has produced so far. In some ways it's more capable, if you actually need those capablities, but those also demand more experience and user input all the way from camera setup prior to taking the shot through image processing in post-production.
The T3i, T2i, 60D use essentially the same sensor and processing as 7D (which has dual processors and a discrete AF chip, to better handle speed and the fancier AF system) so have virtually identical image quality potential.
50D is only slightly behind (15MP vs 18MP, one stop lower ISO, no video capabilities), but many consider it superior in build (magnesium shell) and perhaps a few key features (focus Micro Adjust).
For video, the T3i, T2i and 60D might be slightly superior to 7D... they are newer cameras incorporating a few more video related features, including articulated LCD monitors on two of them. I don't shoot video with my DSLRs, so really am not able to elaborate.... just seem to recall seeing a few things mentioned that there were some video related tweaks in the newer models.
Biggest consideration is probably price.... If the additional features of 7D aren't useful to a shooter, they are paying extra for them needlessly and wasting money that might better be used for an add'l lens or other accessory.
The 7D's AF system is what everyone goes gaga over. And it can be quite useful for particular situations... when it's used right it can make the difference getting the shot or not. But for 95% of shooting, the simpler system of 60D and 50D is almost equally fast and accurate. I have been using a pair of 7D for a couple years now and can nail focus with them probably 96 or 97 out of every 100 shots. With three 50D that I used prior, it was more like 94 to 96 out of every 100 shots. In both cases, some of the missed focus shots were "user error", I'm sure... some were camera-related. So maybe some of the difference is the additional years of practice and experience that the user has gotten under his belt.
I don't use, but wouldn't expect as good AF performance out of the T3i, T2i... They have nine AF points, same as the 60D and 50D, but only the center point is "cross type" on the Rebel/xxxD series... All nine are "cross type" on the 40, 50, 60D. Of course, 7D has 19 points... all "cross type", so offers a bit more potential to select precise point of focus when framing the image (possibly less image cropping needed to accomodate AF point placements and limitations).
7D viewfinder is 100%... one of the biggest and brightest to date on a cropper. But 60D and 50D are pretty darned good, too. T3i and T2i are a bit smaller and slightly less bright, the Rebel/xxxD use a smaller, lighter, cheaper penta mirror instead of a true pentaprism that the more upgrade models use.
There are a number of cool, configurable focus modes on 7D.... a lot more than on the other cameras. When you know how to use these, they can be great. When used wrong, they can cause all sorts of problems (search for "7D Help!" posts here on POTN).
Sure, a magnesium body shell feels good (7D, 50D)... but it's really hard to say if it's any more durable than the more polycarbonate (aka plastic) shells of the other cameras. Metal dents. Plastic can take a pretty hard knock and bounce back, not show any damage at all (Canon lens hoods are pretty tough, for example). On the other hand, what really matters besides appearances is the mechanisms and electronics underneath the body shell... and a polycarbonate shell might hide some internal damage that a dented metal shell would make more obvious.
Internally, 7D, 60D and 50D all use a metal chassis. The Rebel/xxxD models, not so much.
7D's shutter is rated for an estimated 150,000 clicks (same as 5DII). 40, 50, 60D are rated 100K. Rebel/xxxD are rated around 75K. (1D series are currently rated 300K and the new 1DX coming this Spring is getting a new lightweight, carbon fiber reinforced polycarbonate shutter that's rated to 400K.) These are all just estimates, but give some idea of the various models anticipated durability.
7D, 50D, 60D all have 1/8000 top shutter speed and 1/250 flash sync. The Rebel/xxxD models have 1/4000 and 1/200, respecitively.
7D has about the smallest (most precise) Spot Metering of any current Canon DSLR. Only the 5DII offers slightly smaller.
But in the end, it still comes down to what you really need. All the current models are very capable of taking great shots. If you want a travel camera, one of the smaller Rebel/xxxD might be ideal for their smaller size and lighter weight. If you have big hands the larger 60D might be more comfortable, though it's still reasonably light (thanks to polycarbonates). If you want the more full size model and metal shell and can live with one stop lower ISO and no video, 50D might be a great choice, though it's discontinued and rarely seen new now.
The "lesser" cameras have more automation and support for less experienced users... and are simpler to use.
If you have plenty of budget, aren't limiting your lenses etc. to get it, don't mind the add'l weight and necessary user input, are willing to learn to deal with the greater complexity, and actually need and will use the action/sports abilities of 7D, it might be perfect for you and worth the extra expense. Of course, likely there will be a 7D Mark II within the year, maybe a little longer... Which of course will be even more perfect