The Experience Point question has several levels. MHR does have XP, which are usually used to unlock plot elements that has a positive impact on the game. There is some very desultory discussion about changing or improving powers, but like most things in MHR this is more of a plot issue, not a gameplay one. In other words, the question is does the story support the idea of your character gaining new powers, not have you played your character enough to justify the improvement.
Part of this dynamic is MHR's rather consistent bias towards using their own characters in their own re-hashed-from-the-comic-book storylines. This is my least favorite aspect of the game, to be quite honest. I buy the sourcebooks these days to mostly glean ideas, rather than use them in the whole cloth. Since theoretical gameplay is so tied to the Marvel Universe, you're not going to be tweaking their characters.
The other part of the dynamic has to do with the general feel of the source material. To wit: do comic book characters improve with time? I think it has a lot to do with the nature of the character. Comic book characters who started as "teen heroes" like the Dick Grayson Robin, the Peter Parker Spider-Man, and most of the younger X-Men who then grew out of adolescence and into adulthood in the comic book continuity do in fact improve. Kitty Pride is a lot better at superheroics than she used to be.
|The many costumes of Peter Parker|
Another factor is how much attention has been given to the character. "Marquee" heroes, the ones that often define the publishing house franchise, have more often than not been around for decades and usually had their own solo books. Heroes that have consistently been a part of teams and rarely if ever had their own solo series are, by their nature, less developed and less likely to change or specifically improve over time. They might have major plot events surrounding them or even see their powers change in dramatic ways (i.e. Wonder Man) but they don't seem to grow in ability.
All of this takes the back seat to a much greater question: regardless of how the founding material may or may not support the notion, how willing are players to give up the notion of an improving PC? Getting better over time is one of the real constants of RPG's (with the one notable exception: Traveller). People are very used to the idea of having their PC's be able to face increasingly powerful opponents and achieve greater heroics. After a point, a MHR character can only get so much better--he or she can go from a d8 to a d10 to a d12, which represents a Captain America-to-Hulk increase in Strength, for example--but that's it. There's no small, incremental, graduated increase in ability that is more prevalent in other games, largely as a result of the light, abstracted rules set.
Is it a deal-breaker? That largely depends on the group. I think that unless there's a real shift in expectations, MHR won't suit most traditional players for long-term play. We haven't even gotten into tactical versus narrative play, which creates a whole 'nother set of changing expectations.