For years I have been shooting the 70-200 f2.8 IS Mark I and haven't decided to upgrade to the Mark II, because the old lens is still very good and also very reliable.
For shooting events or people you will obviously notice its f2.8 (as previously mentioned by gshocked), but you will also notice its nice bokeh, which should be better wide open compared even to the newer and sharper Mark II version. This alone is the reason I haven't felt the need to upgrade.
The image quality difference between Mark I and Mark II versions is more noticeable using extenders, so you shouldn't use something like a x2 extender on the 70-200mm f2.8 IS or you lose significant image quality. In the Mark II lens this appears to be less of an issue. Then again if using extenders is not an option you want to explore, feel free to ignore that part.
Image quality differences between the f4 and f2.8 versions should be negligible. I have used both and always preferred the f2.8 version. I tend to shoot wide open all the time with such lenses, so the bokeh and f2.8 make for far more than a small difference.
According to the findings at LensRentals.com there is also a marked difference in robustness between the older f4 and f2.8 versions and the newer lens generation.
Just one week ago I accidentally dropped my 70-200mm on a stone terrace. It fell for about 1m and then it bounced from one end to the other. At first I was in a complete panic. Physically I found some dust on the lens hood, but wiping it off, it looked fine. I then ran the lens through autofocus tests for hours on end without finding anything amiss. I am still completely amazed how that came out, because I ended up with the lens showing neither physical damage on the outside nor any damage I could find through my tests on the inside, but I wouldn't bet a pizza on a Mark II surviving such a drop intact based on the findings published by LensRentals.com.