There is one major difference - very early versions of the original cover printing omitted the two appendices (O, P) and have a number of errata issues that were later corrected. So, the 1st-5th printings are shorter than the 6th+ printings. The cover changed with the 8th printing to the Easley design.
When I was just starting D&D, one of my friends had one of the 1st-5th printings, and I was very puzzled to find it didn't actually have those appendices, as I was looking for one of them (encumbrance, IIRC) at the time.
The Easley+ printings are safe as far as having all the content in them!
Cheers!