Unlike previous Marvel RPGs (and unlike most superhero RPGs), the goal of MHRP is to replicate the feel of comic book storylines, with the assumption that you will be playing an existing superhero character in a limited event. (There are rules for going outside that, but it's clearly the default.)
The basic mechanic is for a player to assemble a pool of dice from any relevant attributes (both powers and narrative elements); this determines the scale and effectiveness of their chosen way of affecting the narrative (usually inflicting physical, mental, or emotional stress on foes). They are opposed by the Watcher (GM), who has access to a "Doom Pool" they can use to escalate the situation (and which grows as the players fail). In this way it's similar to games like Fate. Characters can earn XP by meeting certain character or event milestones - these can be used to increase the character's abilities or unlock benefits during the course of the event (such as helper characters).
These basic mechanics are all fine, but there are a few things that bothered me about the book. First of all, there are a lot of ways to affect the dice pool (such as the many power SFX), and it was overwhelming. I actually wish there had been fewer, and the system had been even more abstract. I also wasn't a fan of its apparent assumption that you wouldn't want to make your own material, rather than adapting existing characters and storylines - and their guidelines for making your own stuff were only functional.
Most significantly, the rules explanations weren't always clear. Several times I had to reread sections to get what was intended, and even then it often didn't click into place until I read an example. Had there been more examples at every step, that would have been mitigated. In fact, the chapter midway through that brought everything together should have been at the beginning in a condensed form; it would have provided some useful context for the rest of the book.
Overall, MHRP looks like it could be fun to play, but I'm not sure it's a game I'd want to run. There are a lot of fun ideas in it - the doom pool and the way they handle stress, among others - but a slightly simpler game probably would have served those ideas better. Still, I can see that it'd be appealing to certain gamers, and I won't say it's a bad system at all. Just probably not for me. (Originally posted on Goodreads)