It's highly rated because it's a master of compromises. It manages to avoid a lot of typical pitfalls (bloated, boomy bass; shouty upper midrange, wonky coloration, piercing treble), it offers pretty good resolution and presentation for a closed circumaural, and it does it all for a pretty reasonable price, in an era of frankly ridiculous headphone prices. It doesn't have the cleanest, most controlled bass, nor the most present midrange, nor the smoothest treble, and being a closed design it doesn't sound tremendously spacious or airy, but, unless the listener is particularly turned off by boosted bass and a relaxed overall signature, these are compromises that seem to hit the sweet spot for a lot of people. I personally didn't care for the tonality, and now it's pretty obvious that that would be the case given my general liking for a brighter, leaner balance. But it was the first nice headphone I ever bought, and I didn't really know what I liked at the time.
I don't think it's terrible. I just think at one time in its existence it was over-recommended, with many people who had never tried anything else flooding the recommendation threads with M50 suggestions. Like anything else, it's not going to be the best choice for every single person.