Making one assumption, that you are talking about the CompM 3moa version, here are the differences.
First, the CompM2/ML2 models are built in a new, one-piece housing. The CompM/ML used a multi-piece welded housing. This change was enough to cause some mounts, such as the ARMS #22M68, to not fit. ARMS had to redesign their mount slightly to accomodate the new sight body.
Second, and most important, the new M2/ML2 models use a smaller diode to create the dot. The M/ML models all use a 10 moa diode. In the 3-, 7-, and 10 moa sights the diodes are all the same. In the 3 and 7 moa models the diode is shrouded to make the dot size smaller. Works fine for most but it does have a distinct halo around the dot and, for people with uncorrected astigmatism, makes the dot appear to be odd-shaped. The new models use a 4 moa diode so there is no shroud nor the resultant halo. This makes the dot sharper to most people and even more precise.
There are other changes as well. The new CompM2 has no off position on the switch and has 4 of the 10 settings devoted to NV use. The CompM had two NV settings and an off position. The M2/ML2 models do not have the wire cap retainers. These are now a form-fitting plastic material that keeps them from hanging up on brush, etc. The battery cap threads are different too, so if you have an older M/ML the new double battery cap will not fit. Finally, I think, the new models do not come with any mount. The M/ML models came with a 30mm ring that was too low for most mounting chores and so was most often just left in the box. Now there is no mount supplied with the sight.
There were supposed to be minor improvements in the NV compatibility coating on the CompM2 versus the CompM. I have not been able to observe this difference using a 6015P NVD but it is likely only apparent under particular conditions or with specific NV devices.