This is the original cover of Fantastic Four #1 published in 1961
One obvious difference is the policeman in the back, missing in the reprint, but there are more. Can you spot all of them?
An interesting question is "what is the original cover?". It seems that the one printed in FF#1 has been modified just before printing. The unaltered cover was however put on film, and this was usually the version sent to foreign publishers.
Here is the b/w film of the original cover as was sent to a Spanish publisher (from Ferran Delgado nice blog)