Try to get yourself a copy of the "Munchkin" handbooks. The parody elements do a good job of illustrating many of the basic principles of RPGs.
-Character classes: These are more rigid in some games than in others. Remember, classes can be subdivided by factors like species, profession, political allignment and other variables. In some cases, being a member of a certain class obligates a character to do something, even if the player has no reason to want to do the stupid thing, (such as engaging in a death duel with an ogre for the sake of honour). A healer may not be able to fight. An honourable character may not be able to allow the party to use under-handed tactics, even if those tactics would benefit the player of that character. (The advantage to using an honourable character is that they can be good for in--game negotions. And, while they are negotiating, the other play-characters can sneak off and do the sensibly dishonest thing.)
It is not unheard of for players to make their characters die (often by using a back-door deal with a game's moderator) if they are really unhappy with how things are going.
-weapons/gear: Point balance. Treat weapons as a sub-category of gear. The effect of a given items should be reflected in its in-game cost or the difficulty in getting one. In theory, an expensive enough item (call them "dungeon master's dice") can do it all.
Characters can grow out of combat based on either in-game plots or the player's whimsy.
Location is critical. Characters have to be in proximity for close-quarters fighting, and may have to spend "moves" getting close enough to fight. (This is why markers or miniatures are so important.) In combat, the general goal is (in the words of Munchkin) "to kill the monsters and take their stuff".in combat, does it generally matter *where* your characters are? Is it ever advantageous to surround an enemy, or somehow lead them onto terrain that works against them?
A moderator (or dungeon master) can be really nasty with this. They can set up tons of weak enemies, and have the players finding useful items scattered about. These items can be everything the gaming party might need (cursed weapons, enchanted armour, healing potions, stuff to make players immune to the curses on those otherwise useful weapons mentioned above, tons of gold for buying more stuff later....). And, the the moderator can have a monster jump out that puts it all in perspective. Those weak enemies (bandits of various species, maybe even some wild fauna) were not running towards the players, they were just trying to get away from the big monster. All of those useful items, (the stuff that seemed like it was made for a dungeon raiding party), were actually things dropped by members of other parties while they frantically tried to get away (and maybe failed to get away).
Even the most Munchkinly power-gamer will get bored if the game is too easy. So, enemies and problems are essential. Try to work out ground rules for "how bad it can get" beforehand though.
-Dice tossin' Dom.....