Here is what they said (mainly regarding GAME mechanics):
1. A Goblin with a short bow (and a lucky GM) can be far more dangerous than 2 Minotarus closing in! 
2. Combat is far more interesting in RM, with Criticals, Fumbles.
3. However, if you suffer a critical, you might be put out for several rounds (One PCs suffered 3 or 4 stun rounds and couldn't recover), which is a drag.
That player (who was basically a fighter) didn't like it at all.
4. Instant Kill is possible (one PC dropped a Minotaur with a lucky roll). They all laughted in delight when it happened, until they realized I'm using the same tables for the Monster attacks!
5. Working as a Group is very important.
6. Spell casting (with the modifiers, preperation, RR and all) can be complicated to resolve (many rolls with lots of modifiers).
7. But - the spell lists are very rich, and provide "more to work with".
8. Many great player options in building his PC.
9. The math is not an issue.
10. Healing is a big issue (they didn't have a healer in the original group, they relayed on potions etc).
1. There are more extreme ranges of luck than in many other games (although rolling 3x20 on an attack can instantly kill anything in 3.0 or 3.5,) but it's very rare that something like that happens and you can reduce the risks to the point where it's not something to worry about.
2. Yes, although I would say that initiative system is what makes it more interesting than D&D.
3. Explain what happens to him, don't just tell him "you're stunned for x rounds." A stun doesn't mean you're out though, you can still act just not cast spells or hit stuff with your pointy stick. When you're stunned, you're basically rocked but not out.
4. Coming from a "Save or Die" system, this shouldn't worry them.
5. Agreed, for example you should let the lay healer and magician take care of the remaining minotaur while you check the treasure chest behind him.
6. This is probably the biggest change from 3.x to RM, having to actually succeed at casting the spell rather than it just working automatically. Make sure the spellcasters read through the necessary rules so they fully understand how it works. Since they'll spend 2-3 turns on a single spell, it'll suck to hear that it doesn't resolve as they thought it would once they actually cast it.
7. If you have any munchkins in your group, their eyes probably lit up when they saw the massive Spell Law content.
8. Yes - which is why I hate converting from "less detailed" systems! Give them the option of rerolling, please! If you have a campaign then you could say that they can reroll, but with the same concept as before (or "remaking" their old character.)
9. For some reason, our group has massive problems doing complicated calculations like: 17+4+2-5
Usually the calculator is brought out to take care of it.
10. I disagree. You simply have to get rid of the old fantasy game cliches you have in your head and look at it from your character's point of view. Keep yourself protected, overwhelm the enemy, strike where he is most vulnerable, etc. Something really important that we keep forgetting to do is moving in and out of range of enemies. Forcing a conflicting action roll to even be able to attack is a great weapon and denies the enemy the chance to use full attack or acting in the deliberate phase.
You simply don't want someone to strike you first, or at all, ever.
The only issue I had is with a player who feels that:
1. His lack of knowledge in the rules will prevent him from building an interesting PC (he likes to find what he calls a "Build", meaning a PC that excell in one or several areas).
2. You might go down in combat, or suffer a too dire injury to contribute, and while the group carries on one player feels useless.
3. He felt that while RM has more options for a fighter (in a battle), he had a hard time with all the numbers involved (penalty for movement, OB/DB split, Parrying limitations, tracking damage, bleeding and stun).
1. Making a build should be discouraged in my opinion, unless he can produce a good character concept for it. It's not nearly as good to optimize a small range of skills in RM as in most other games though. Builds work best in games that have very limited but diverse options.
2. This is something I've heard too, but I'm not sure what the person is thinking. Read it slowly:
"You might go down in combat" - yes..? If there was no chance of going down in combat, why even play it? Just skip it and say "you defeat <insert opponents> and move on."
"or suffer a too dire injury to contribute" - this I understand a bit better, since it's coming from a D&D player and D&D doesn't really have any negative consequences of taking damage before you go down from them. Ask him if he can suggest a good alternative, or maybe suggest to him that he can skip all stuns and other injuries, but will take extra hits instead. So he'll be able to fight until he's out of hits.
I think this mentality is the result of video gaming, but to me the interesting part of playing a RPG is that you can't always just keep trucking, fighting everything you see and know that your character is practically immortal. Others feel that to have fun, they want to develop the same character for years and that means they will have to be protected, either by the GM or the system itself.
3. There's a lot of bookkeeping for sure. That's simply in the nature of a detailed game.
