Rolemaster is in many ways a more complicated version of D&D.
Learning spells for example, is done in list. In the old system, you had to roll % based on how many ranks you purchased in the spell list so unless you maxed out those ranks, you might be advancing without getting those spells.
Much larger list of skills and in some ways, leads the way to D&D with it's class/cross class skill set in that a profession is good at certain things but not others but made much more complicated by having whole ranges of point differences. Made this very difficult for a GM to track a player's skill point allocation.
Stats were broken into temporary and potential. the former represented your stats as they currently were and the potential as they might one day be. Based at time of character generation and could even go down due to poor dice rolling. Much better than D&D in my opinion as even with it's current idea of advancing stats 1 point every 4 levels, your character still remains fairly static.
Tons of professions. No feats per say, but background options could act as such initially. Background options were based on the power of your race. A common man might start with 6, a huge amount as you could spent them for skill bonuses, items, and other goods. A high man (i.e. Dunaden from LoTR), might get 2. Other factors weren't really a consideration in terms of game balance so if you spend those background points poorly...
Races: Lots of interesting stuff ranging from the standards to the High Man and one of the first times outside of MERP I'd seen such a race presented.
Magic Items: Someone said that RM was more realistic. I laugh at them. Some of the magic items in this game system are outrageously powerful. Without even having any of the books anymore I remember the Pulverising Fist of Maar simply because I adoped it after cutting it down a bit. Magic items in the old system were known for being outrageous. The RM Companion with the intelligent weapons was one of my favorites and perfect for doing an Eternal Champion/King Arthur style game with it. Spell point multipliers were also great as the game used a power point mechanic for spells and psionics.
Combat: Here you either have to wade through a ton of options or trust your players. Cross referencing criticals with larger creatures could be something of a pain. Characters even at higher levels can be squishy.
XP was a nightmare. If there's one thing I wish that D&D 3rd ed hadn't done is steal the idea of a player level versus a monster level. XP also wasn't very clear in that you got XP for hit points inflicted to the enemy and yourself. XP for different values of criticals, which decremented from the monster's base XP. Xp for killing the monster, which resulted in a lot of 'heroic' attacks as players tried to outdo each other and claim the full victory. XP for travelling. Xp for performing maneuvers. Nightmarish. On the other hand, the smart thing D&D did is steal the concept of one xp table for all the classes.
Rolemaster is not a simplier system. It's more detailed in some aspects and more humourous in some but realistic? No, getting your head lopped off by a fantasy goblin with a rusty battle axe is not realistic but could be funny depending on how many feet it bounces.
