The critical hit system does not mesh well with the hit point system and the tables are far too erratic for my taste, but the descriptions provide such delightful detail that I perpetually come back to them, try to cook up a way to integrate them into whatever I'm playing at the moment, and give up in disgust.
The second thing I love about Rolemaster are the Angus McBride covers (for the second edition, later reused for the Rolemaster Standard System).
The ruined city on display on the cover of the three main books is my personal Skull Mountain, i.e. it fires my imagination like no other illustration of a fantastic location. If I ever design a megadungeon of my own this is it.
Finally, the rules - and I mean all the rules, e.g. the many Rolemaster Companions - form a huge, baroque encyclopaedia of fantasy roleplaying (of a particular kind, I admit).
It's impossible to use all the options, not only because many are mutually exclusive but mostly because an already complex game would collapse under their combined weight. Also, many options aren't even particularly well thought out or useful at the table -- I suspect many were never tested.
Just how useful is a table that tells you how much your character's Weather Watching skill improves for every 1000 years of age? This sort of thing is a prime example of silver age obsessions. It's not useful at the table but reading this stuff or - God forbid! - cooking up similarly byzantine subsystems can be immensely inspiring. These days, I'd rather get down to actually playing but I'll always have a soft spot for the treasure trove of ideas that is Rolemaster Second Edition.
My longest running and smallest campaign (five years and two players, respectively) used Rolemaster Second Edition and conjures up nothing but fond memories. Great times!