The actual mechanics of each RM edition are very similar and simple. There is some variation, but generally it works as follows. A bonus for an applicable stat(s), along with a characters expertise with a skill, weapon or magic, is added to a D100 roll, minus any negative modifier for circumstances, defenses etc. The result is cross indexed on a chart and that's it. There are times when a 2nd D100 roll is required to determine the results of a critical hit, a fumble or some other circumstance.
Many gamers are put off by the "optional" complexity, the adding of numbers that may go over 100, and the necessity to reference a chart. But after years of playing and GMing it, I found a combat round to take no longer than in D&D, assuming that both the GM and players are familiar with the rules and their character.
Character creation and advancing class levels is a lengthy process, but I've found the finished results to be very rewarding and worth the effort. Combat can be quite deadly if the player doesn't learn to parry, and most GM's have some kind of house rule that helps the player to avoid dying on an unlucky strike from an enemy.
As for the various editions ...
- HARP is often referred to as Rolemaster-Lite
- Rolemaster Classic is a re-printed and edited version of RM 2nd edition which offers fewer options than the Rolmaster Standard System or Fantasy Roleplay (RMSS - RMFRP) which have oodles and oodles of options, traits and classes.
I love playing and GM'ing RM, but readily admit that its not for those looking for a simple, stream-lined and basic game.
I hope that helps.