I've only been able to glean a few things from him and Wikipedia. Namely:
1) Once you finish character creation, it's just a roll of percentile dice + modifier = outcome to anything.[/quote
Yes, but there are a multitude of resolution systems.
For combat (both weapon and bolt/ball/cone attacks) you look up the result on an attack chart. Many results then require you to roll a critical on another chart (the Crit Tables for which RM is deservedly famous).
For some non-combat manouevres, you look up the result on the Moving Manoeuvres chart, which in turn yields a result of Fail or else a number between 10 and about 150. This latter number can in turn be interpreted in various ways depending on the action attempted, but normally represents something like Percentage Success. The MM chart itself has one column for each degree of manouevre difficulty.
For some non-combat manoeuvres, you look up the result on the Static Action chart, which has entries at below -25, -25 to 04, 05 to 70, 71-90, 91 to 110, 111 to 175, and 176 and up. Each of those entries tells you the outcome of your manoevure, from terrible failure (at -26 or down) to utter success (at 176 and up). Each sort of manouevre (picking locks, stealth, crafting etc) has its own Static Action chart.
For attack spell-casting other than bolt/ball/cone attacks there is the Base Attack Chart, which generates a modifier to your enemies saving throw (called a Resistance Roll in RM).
Depending on edition, spell casting may also require a manoeuvre prior to the attack called the Extraordinary Spell Failure (ESF) manouevre in RM2 or the Spell Casting Static Action in RMSS/RMFRP. This is resolved something like a Static Action with special modifiers unique to spell casting.
2) Charts for everything. EVERYTHING. Charts charts charts.
As I've just indicated, this is not untrue. But it's also not as bad as it's often made out to be. A couple of things have to be kept in mind:
*First, a good RM GM has an instinctive feel for a lot of the charts, so for many manouevre rolls which are not utterly crucial (eg sneaking past the guards on one's way to a shady deal in the bad end of town) the GM can just adjudicate from the result of your dice roll plus modifiers without needing to look up the chart.
*Second, RM works best when the chart-intensive action is confined to key moments of the game. If you try to run RM with as many encounters per level as D&D it will bog down into a chart nightmare, as encounters (whether combat or non-combat) that are meaningless from the plot point-of-view hog an inordinate amount of playing time.
I think that the above two points should be seen as something like an informal analogue in RM play to the difference between simple and extended contests in HeroWars. If you wouldn't do it as an extended contest in HeroWars, think twice about launching into it in RM.
3) It's "realistic". For instance, while there are "Hit Points", most injuries come from a "Critical hit" table, and the degree of those injuries usually designate you dieing. And even a skipping stone can kill you with a lucky roll.
This is true. At mid-to-high levels (eg 10th and up, which plays a little like 6th and up in D&D), healing magic is normally good enough that the risks are fairly minimal and can be handled by the party. At low levels you will either need the GM to fudge, the players to have Fate Points of some sort to force GM rerolls, or be ready to introduce new PCs after combats. In the past I've tended to fudge a bit as GM; in the future I'd use Fate Points.
4) "Magic" is in three categories: Arcane, Mentalist, and Channeling.
"Arcane" is called "Essence" in RM - it includes Elemental magic, Illusions, Item Creation, plus a lot of auxiliary stuff like Flying, Teleporting and some Shape Changing. Channellling is what D&D calls Divine, and covers Healing, some of the best Divination, the RM equivalents of Fabricate, Create Food & Drink etc. Mentalism is psionics - self healing, self buff, divination.
5) You're just a vulnerable kitten without armor.
Generally true unless you have what RM calls Adrenal Defence, which is the functional equivalent of the monk AC bonus in AD&D and 3E.
I'm interested in Mentalism, but I don't know what it can "do". Is it telekinesis, telepathy, can you do some sort of force-ninjitsu to be Neo, what? I'd enjoy something like a Jedi, or in D&D terms, a Monk/Psion. But I just don't know what the options Mentalism offers.
Mentalism might suit. For a Jedi you actually need hybrid Mentalism/Essence, which is a little limited in core RM but handled well by the near-infinite number of supplements.
Aside from the three magics, and obviously "I'm a warrior with a sword", what other "Classes" or Archetypes does Rolemaster facilitate?
A very wide range. In my view the far-and-away biggest strength of RM is its rich and flexible character build system, which allow (especially by D&D standards) a very nuanced and detailed expression of the personality and capacities of the PC.