Nothing.
I did a lot of research (couple year's worth) before buying the M2. The TAZ was the only other printer to make the very short list.
I considered the TAZ because it had a larger build volume, which would have meant I could print bigger things, although from the standpoint of quality, the M2 was
way ahead at the time. Ultimately I decided on the M2, and I'm very glad that I did, because printing something that fills up just the M2 build volume
can take up to 10 days! The limiting factor is the speed that you can print the particular filaments, not how large the build volume is. And I figured I wasn't going to need to print much that ran over a week.
Some of the other machines tout things like auto-leveling, which in my mind is not actually an advantage, if you have to wait while the machine runs through a leveling process
every time before it starts a print. It's much better in my opinion to have a machine that just keeps it's level between hundreds of prints, like the M2 does. (Set it and forget it.) Many of the other machines don't keep their level, and the results are sloppy.
Other printers look fancy and have a nice enclosure. That's aesthetics, unless you are printing ABS, which requires an enclosure to keep the temperature stable enough to not crack on tall prints. (If you're going to print ABS, an enclosure is needed.) So for if you intend to print a lot of ABS, it's a good idea to get one with an enclosure. (I had no intention of printing ABS.....it stinks. Husband is unfortunately allergic to everything.

)
And as far as the TAZ goes, I'm sure it's a good machine, but if my memory is correct, we've had guys here on the forum who switched to an M2 from the TAZ, and liked it better.
Of course, if I paid that much for a machine, I'm sure I'd tell people how wonderful it was too!

But truth is, aside from the larger build volume, the M2 is probably more stable, easier to use, and will give you better prints. (The problems with blobs, getting the filament to stick to the build plate, getting an even extrusion rate, getting the gap set correctly.....those are experienced by
every 3D printer. It's part of the manufacturing process that we just have to figure out how to work around if we want to get a good print. The only thing that spits out a perfect plastic toy with the press of a button is a gumball machine.)