I know this is an old thread but in case anyone still has this issue, especially as we have to deal with higher and higher resolutions, depending on the complexity of your comp, try this:
1) File > Scripts > Scale Composition.jsx
2) Chose New Scale factor of .5 (or .25) [Scale]
Now you should be able to zoom in farther. When done, repeat Steps 1-2 but choose a scale factor of 2.0 (or 4.0) to reverse everything.
Hope this helps!