From the entire RM collection (taking RMu with a grain of salt as it has not been completed) I love the expansiveness. It's more a "role playing simulation" than it is a "table-top game." The ability to customize is staggering and while some people see that as a downside, you can easily restrict portions, professions, skills, spell lists, etc. to make the game manageable. Saying it is too vast is a cop-out. You can easily tailor everything to suit your style, setting, intentions, etc.
Specifically from RM2, I love that I can use a Companion such as Elemental Companion to truly customize the game world (or At Rapier's Point or Oriental Companion) and base the entire game setting on a single Companion and magic system. I love that Shadow World provides an already built world to jump into, or allows you to take portions from those SW books to inject into your own world or to even play as a stand alone adventure for a small group of new players. I've said it dozens of times in other posts, but I believe EC's handling of elements, elementals, elemental realms, creatures, etc. is sheer brilliance. It's hands-down my favorite book from the entire collection.
I love the number of spells available to choose from as opposed to a single, small collection of spells that all spell casters have access to.
I love the variety of Professions. While the Professions available allow players to choose their own special archetype, the PC still has the opportunity to learn nearly any skill, at a cost. It's not like choosing a class and the choice automatically eliminates some skills from the PC. There is a built-in balance there; several ranks in the low cost skills, or buy the couple of higher cost skills to really add flavor to the PC, or drop 20 DP for a non-spell user to get a 5% chance to learn a spell list. (I had one fighter PC do that and he finally learned a spell list at level 6. It was a small selection of spells, but for a fighter to be able to cast spells was a great surprise for the bad guys.)
I love the PC creation mechanic. Every PC is different . The stats are different, the bonuses are different, the background options are different. The background options really add the greatest flavor to a PC. The number of background option lists to choose from is awesome too. I've had some great fun with PCs based on the results of the background options, even the Chaos Tables were fun.
When I first started playing RM, the GM at the time was also using MERP books in addition to the RM books so I can't say which aspects of MERP I like even though I did have some experience with it. When I asked the GM about using both sets of books, he stated he liked the way some weapon tables were and some game play mechanics.