It is not your body but the aperture limitations of your lens unless you are shooting in very good light. Soft pictures are due to inadequate shutter speeds. Unless you are shooting in very good lighting always crank up your ISO setting and then go buy a cheap prime lens like the 50mm 1.8 and learn how to shoot in aperture priority mode. Not knocking the 28-135mm but get a lens that lets you shoot in the f1.8 – 2.8 range and I would say your 28-135mm will start gathering dust. The only problem is that the cheap 50mm 1.8 will expose you to what good optics can do and then your addiction to good glass will most likely begin. It can become a very very expensive hobby in a hurry.
And yes the rule of thumb is 1/focal length but when you are shooting a crop factor body like the 50D you have to factor in 1.6 x your focal length. So if I was shooting a Canon 50D at 100mm I would want a minimum shutter speed of somewhere in the 1/150th or 160th of a second.