Picantinny rail and new grip design on the A1. Mechanically, they are pretty much identical. The new A1 should come with the new extractor design that improves extraction. The original M will probably come with a manual safety.
Slightly different grip between the two, the M-A1 includes a rail under the dust cover. The original M had a manual safety in addition to the trigger safety, whereas most of the M-A1s imported in the US lack the manual safety.
Some of the M-series had a semi-matte black slide finish whereas the M-A1s have more of a parkerized slide finish.
The rest of the gun is the same -- internals, operating system, barrel, and slide, and magazines.