Fallout 2 (the only one I’ve played) comes about as close to a true pen and paper RPG as any PC game I have seen yet. Character creation is entirely non-random, you have points you can assign to stats and you can choose three skills to specialize in. You can pick a couple of special traits at character creation that give your character special advantages but with drawbacks - i.e. if you choose Small Frame you get more action points but you can carry a lot less, if you choose Fast Metabolism you heal faster but are more susceptible to damage from radiation or poison. After the game starts you get to choose ‘Perks’ every few levels, which are bonuses that don’t have drawbacks. These vary widely, from gaining an extra level to having particular combat advantages to being a ‘Kama Sutra Master’. Many of the perks require certain prerequisites - to be a Kama Sutra Master you have to have a certain dexterity and endurance, and I believe your character has to have had sex but I’m not sure on that one.
Combat is turn-based and rather detailed - you can make aimed shots that increase your chance of getting a critical and have other, non-HP related effects on the opponent - you can blind a creature by aiming for it’s eyes, shoot it in the arms to reduce it’s chance to hit, or if it’s running away from you popping it in the leg can slow it down. The armor system is well thought out, armor protects against different kinds of damage differently, and in three ways - it can reduce damage by a fixed amount, reduce damage by a percentage, and reduce the chance that you are hit for damage at all. There is a wide variety of weapons, ranging from primitive melee weapons to real-life modern weapons to futuristic ones.
Conversations are far more important than in any other CRPG I have played - the choices given to you in conversation are based on what your character has found out or done in the game, what your skills are (i.e. if you have a high Science skill you might get more options when talking to a scientist), what your reputation in the game world is, how effective a liar you are, what your intelligence is - basically conversation can have just as many factors effecting it as combat. If you try to tell a lie, it will roll against your Fast Talk skill, if you have a certain Perk you can talk about things you don’t really understand (effectively getting the same conversation options that you would get if your Intelligence was a few points higher). If you make a Charismatic, Intelligent, Fast-Talking character you will get a LOT more out of talking to people than if you make an unlikable dull person - you can talk your way out of fights, get people to do things for you they normally wouldn’t, etc. What you say to people will effect your reputation, if you are rude to the wrong people word will get around and people will react differently to you.
The game also has a detailed reputation/karma system. Your reputation is based on what people see you doing. If you help people out a lot in one town your reputation will increase and people will treat you like a hero eventually. If you behave villainously the opposite will happen. You can outrun your reputation to an extent, but word gets around - if you stick around in one town for a while doing good deeds, you may see that in neighboring towns you haven’t even been to yet your reputation is increasing as well. Likewise, you may do some really evil stuff and then skip town, and be OK for a while in the new town, until word starts getting around how you acted in the last town, I’ve actually had a character enter town and have the townsfolk be OK with me, and then the next day they are hostile to me because they heard about what I did in the last town. Karma is somewhat like reputation, except that your acts don’t have to be witnessed to effect your karma, and it follows you around. It doesn’t have as big an effect on how people react, but if you have a really high or low karma it will effect how people treat you regardless of your reputation. Many characters can sense your karma more than others - in one game I did lots of good acts, then travelled far south to a town where nobody had heard of me. I tried to get some training from an evil kung-fu master but he could tell that I was good and refused to help me. There are also special little Karmic Titles that you can get for doing certain things, like Grave Robber or Child Murderer.
The game is also very open-ended. There is very little that you actually HAVE to do to finish the game, and tons of side-quests. Many of the side-quests can be completed in several different ways, with different results. If you’ve played the game before you can use player knowledge to go pretty much straight to the end-game, though at your low level you won’t be able to finish it. It’s open-endedness frustrates some people more used to linear RPGs, they don’t know what they are supposed to be doing, you have to realize that you do what your character WANTS to do, and no matter where you wander you will pick up information, experience, and gear that will help you finish the game. You can play it like a more linear game, go to the first town, finish all the quests, then go to the next town you have heard about, but you definitely don’t have to and I usually don’t. You can also play the game as an evil character, though a majority of the quests seem to have a good slant to them and you get a good deal of your experience from finishing quests.
NPCs that join your party are not controlled directly as in Baldur’s Gate - you can give them general orders from a menu (like stay close to me, only use burst fire if there’s no chance I’ll be hit, and use healing kits when you are seriously hurt), but some of the menu options are not allowed for certain characters - you can’t order the timid trader to wait until he is about to die before healing himself, nor can you order the crazy tribal to run away at the first sign of danger. You can finish the game without any help from NPCs at all. Some of the NPCs are almost completely worthless (there’s an opportunity fairly early on to get married, and your spouse isn’t really good at anything but getting killed - you can always sell him/her (or any NPC) to a slaver though).
The game has a lot of dark humor in it, and tons of little pop-cultural references. It’s got a few absurd bits, like one character that will explode if you keep fucking him repeatedly, and more than a few Monty Python references. It’s quite possible for your character to get addicted to drugs, and depending on the drug this can be very bad for your character, though it can lead to interesting character development (one of my character’s first priorities upon entering a new town was to find a dealer for her particular addiction, otherwise she suffered serious penalties).
From what I hear, Arcanum is a lot like the Fallout RPGs, but with a different setting and even more detail.