Short: It successfully applies every lesson learned from previous superhero mushes in a well run fun way to tell your story your way in a sandbox environment.
Medium: Heroes Assemble Mush merges DC and Marvel Continuity with a few inclusions from Image and Darkhorse; whilst avoiding the genre dilution of other shows such as the Dresden Files, Buffy the Vampire or the Last Starfighter. It uses a semi automated log system to allow anyone to read role-play logs with scheduled events so that there is something going on practically every night, and a flexible and fun plot system to connect the various events together. There is (as of now) very large player base which gives the opportunity for fun and social RP to allow you to play just about any character; feature or original with a few sane limits put in place.
Maximum Verbosity: Up until now, I've largely written about what didn't work in mushes and this is my first positive review of one. I've been doing super hero mushes since Children of the Atom and I played my 2nd version of Grenademan (after first making him in Torg) and had an absolute blast. The biggest problem with an X-men themed mush is the Mansion problem where 2/3rds of all meaningful play takes place in the X-mansion. Heroes Assemble solves this by mushing everything together (Gotham/Metropolis/New York City) are all in one area where everyone hero and villain alike are in close proximity.
Another big problem a lot of mushes have is how to handle Original Characters. This is complex, because most who want to play an original want to be the hero of their own story, whereas those who tend to play feature characters most want to play variations of the same stories they've read about in the comics but have them 'in the shoes of' said hero. Heroes Assemble does this by finding the sweet spot between restricting too much power to start with (no reality benders, no casual time travel, etc) as well as ensuring that they have to have room for growth by saying that they can't be more powerful than Captain America. that's not to say you can't play someone stronger, faster smarter etc...but Captain America is insanely powerful in many ways; he has an indestructible shield, is insanely popular, is among the best fighters in the world, and has unparalleled leadership while also being super strong and super fast. He is a solid medium level character which lets an original start out as robust; but not stronger than Superman and smarter than Tony Stark. But unlike feature characters, the staff are very very welcoming at allowing Original Characters to grow over time with some becoming members of most every super team, insanely popular and also very powerful. Its one of the most difficult problems in running a super hero mush and they do it well.
the plots/logs/and events system is masterfully coded and dutifully manually uploaded by staff from the databanks of the mush to the Wiki every night to allow people to share in the fun of a scene almost like ten based comic books. As of today in the four years the mush has been open, it has done over 18200 scenes, which when you break that out by weeks of operations means 91 scenes a WEEK for 200+ weeks. That's pretty impressive. When I count the number of scheduled scenes (11) during the summer slow season no less; there are plenty of things to do and setting something up is as easy as pie.
I ultimately left, for a lot of reasons. At the end of the day, my plots weren't just interesting people. People want a certain level of genre normality and I felt that my ideas didn't fit their expectations and got sick of setting things up and having no one sign up for them. Cliques have been part of our social experience since high school and a mush is no different, but what I can tell you is that to the extent that a mush can be structured to overcome that problem, Heroes Assembled has done its absolute best to handle this problem. You can't permanently change the setting; the illusion of growth is just as much of a problem in a MUSH as it is for the genre. By allowing everyone to tell their stories, it ensures that no one else's story can affect everyone else but that still leaves a thrilling range of tales of what you can do. If you want to play a super hero in a text based medium; this is the place to do it.