Wow. Fantastic thread and really great replies! I'll throw my 2p in as well.
A lot of the RM players I have now were AD&D players first and the cardinal sin they make is that spell casters in RM2 work just like AD&D. When they first start playing RM, I encourage them to choose a non-spell caster to get used to the system first and to see how it works. The die-hard D20 players always try to argue with me and they inevitably roll up a very poorly crafted RM spell user. Any long time player in RM will know a poorly crafted spell user; high stat values in the wrong categories, too much emphasis on non-magical skills, one or two spell lists even at level 3 or 4 PC level, spell lists from non-class spell lists, on and on.
Then three's playing the spell caster "less than ideally" in game play. They remember back to the days when D20 spell casters had 1D4 HP and they see their RM spell caster with 30+HP but they don't parry or hide, they don't realize that "2 rounds to prepare, 1 round to cast" is a long long time in RM combat, and Stunned crits are devastating in RM. Spell casting in RM is a lot more involved than in other gaming systems I've played and spell casters tend to be weaker at lower levels unless really crafted well.
Compare that to the long time RM players who know how to make a really good spell caster. They have 4 maybe 5 spell lists at level 3 or 4, they've maxed out their magical skills and spent some good points in a weapon and maybe some defensive skills. They know to plan their spell casting with the flow of the game. We've had some amazing spell casters, but they weren't the majority of the party. We would have maybe 2 spell casters in the group at most at any time. One time, we had a non-spell user who spent 20DP for a 5% chance to learn a spell list. He eventually rolled 96+ and learned the spell list!
Our gaming world is a little different as well. We start all of our players at level 5. One of the main reasons was to give players a chance to make really well developed spell casters and to give them some longevity. This also allowed them to cast level 1 spells in 1 round a little easier for them. To help with the massive amount of spells lists, we limited the magic to only Elemental Companion (RM2). Only Elementalism is allowed and I've since added Alchemy Companion to the mix as it fits perfectly with the "Age of Discovery" in which our gaming world takes place.
Something I've learned over the years "Protect the Healer/Spell caster!" Some members of the party would invariably protect the spell caster while he prepares his spell. I've found this isn't quite as imperative in the D20 system. As the RM spell casters go up in level they can be very powerful and it's the party's best interests to keep them alive and to allow them to cast spells. We learned over the years to be very careful when spell adders and PP multipliers as loot. Spell users really became powerful then.