D&D Unleashed, part I: YES YOU CAN! (Classes)

Once upon a time, I got tired of OD&D/BD&D and all of its limitations, specially those regarding classes. "Wizards cannot use swords", "thieves cannot wear heavy armor", or even the (misguided) "non-thieves cannot hide", and so on. I thought this was silly. To this day, I dislike the limitations some editions of D&D try to enforce (even if they say you can take a feat to ignore it, although this makes things better), but once I better understood them, I realized that leaving D&D because of them is misguided.

Instead, I got rid of the limitations.

Of course, lots of people have done this before. Is easier than you may think. Let's see...

Can I use any skill, even if I'm not a thief? 

YES, YOU CAN. This is the easy one. In fact, this was probably the case from day one, as many have argued. Thief's skills are meant to go above and beyond a normal person's abilities. For examples, a thief can hide "in shadows" and climb "sheer surfaces", feats there are impressive for most people - but almost anyone can hide behind a curtain or climb a knotted rope. The thief's ability to "hear noises" is a bonus to what all classes can do, and there is no reason not to apply this to every skill they have.

Can I use any weapon, even if I'm not a fighter? 

YES, YOU CAN. And it won't break the game or be unfair. Fighters will still make the best of whatever weapons they get their hands on, due to better Strenght, HP, BAB, THAC0, etc. In OD&D there was a specific reason for this limitations: magical swords were the most powerful weapons around and were quite common in the random treasure tables. If this isn't the case in your game, there is probably no good reason to keep the limitation.

Now, if for some other reason the sword is simply the most powerful weapon of the game and everyone will use swords if you don't stop them, the problem is with your weapons, not with your classes - remember that older versions of D&D would sometimes use weapon speed and effectiveness against armor along with damage, so the "best" weapon was dependent on circumstances. But even if your swords do 1d8 damage while all other weapons do 1d6 and you don't use other distinctions, there should be plenty of "narrative" reasons to try other weapons: concealment, religious taboos, prohibitive laws and customs, availability, etc.

So what about armor? Can I wear any armor, too?

YES, YOU CAN. This one is a bit trickier, as armor, unlike weapons, can be equally useful to all classes. While I find unreasonable that wizards and thieves cannot wear armor, saying that armor gets in the way of magic and thievery sounds good. A penalty or a flat chance of failure due to armor sounds better than prohibition. Or you might want to use something from the next paragraph...

Anyone can read spell books...

Oh, yeah? So what about spells? Can I cast them even if I'm not a wizard?

YES, YOU CAN. And not just for scrolls, either. You can use spell books if you want. Look at thieves again: 10th level thieves can use magic scrolls with a 10% chance of the spell being reversed (if 7th level or greater), in OD&D. Yes - they are that good. Now think what would happen if someone else would try that - the chance of failure would be way greater.

For a one-line old school solution, save vs. spell to avoid the spell turning against you or causing other disastrous effects. Not old school enough? Then use the "% Chance to Know any given spell" in Supplement I. By the way, the books described in Supplement I are good examples of stuff that may happen when one reads the wrong book.

(And did you ever notice how magic-users are harmed by the Book of Exalted Deeds and not by the Book of Vile Darkness? What does this say about the nature of magic?)

Okay, now I guess you'll say I can turn undead and use cleric spells as well?

YES, YOU CAN. If you faith is strong enough, there is no reason not to. Sure, you have to respect any taboos that a particular god may enforce (avoiding particular weapons or arcane magic, for example), but a deity won't abandon its followers just because they aren't part of the clergy. Clerics can probably get more leeway, as a life of righteousness allows for a little misstep every now and then... or at least this is what they tell themselves in order to be able to sleep at night, depending on how gritty is your game.

By the way, you might need a cross to turn a vampire, depending on your GM. Better be prepared.

What about elves, dwarves, monks, barbarians, paladins, etc? Can they do anything as well?

YES, THEY CAN. Same ideas apply. Monks can wear armor although it gets in the way of their kung fu. Paladins have got it mostly right, they are just fighters that gain special powers in exchange for following some tenets. Dwarves and barbarians can cast spells, but they know it might be a bad idea, specially if they have more wisdom than intelligence... And so on.

... but results may vary.

And what about 5th edition? 
The way 5th edition works might make things a bit different, but not much. For example, BAB doesn't vary across classes (is usually equal to proficiency), but the differences in STR, HP and special traits should be enough to guarantee that a wizard with a sword won't break anything or get in the fighter's "niche". A rogue with a heavy weapon is fine too; she's probably better off with a rapier anyway. Same goes for spell-casting fighters and so on.

What's the point?
Limitations in D&D are a matter of taste, but if you have a player that wants to create a wizard with a sword, why not let him? A fighter of the same level will still beat him in a sword fight. Choices are what make RPGs so cool, and it will be fun to see what happens when the fighter wants to cast a spell, without knowing what is waiting for her...