- No skill system at all -- use ability checks with appropriate modifiers
- Secondary skills only (as per 1st edition)
- Rules Cyclopedia skill slots
- 3.x skill points
- 4e trained/untrained skills
- The Adventures Dark & Deep system, based on a prior OGL creation, which involves the purchase of skill ranks with experience points, the skills themselves use a modified ability check system.
At first blush, the "no system" has a lot going for it. There is something liberating, frankly, about not having a skill system and just winging the outcome with your players. Obviously, this requires a level of DM flexibility and skill, as well as the trust of the players. It also requires consistent application. Over the course of a campaign, the flexibility/"winging it" solution ends up with loosely codified houserules using a variety of modifiers to either the target number or the die roll itself.
My main beef with this (as well as the 1st edition solution) is that it does not allow a PC to excel at a particular skill. How do you simulate arcane knowledge, tracking, or even being a great horseman in Basic D&D? Clearly, not everyone has this knowledge -- so you end up with a houserule that says some ability checks are only usable by certain classes. This creates a de facto "class skill" system where some skills are only usable by trained individuals and others (like stealth, say) are usable untrained.
On the other hand, there are other cans of worms opened with a full-blown skill system exported to pre 3.x D&D. Many of the rules mechanics (surprise, finding secret doors, falling into traps, etc.) are handled on a flat 1 in 6 or 2 in 6 probability, with no distinctions between different races and classes of PCs. Once you inject perception, athletics, and other combat and non-combat skills into the mix, you are fundamentally altering how the basic game works -- and not just for the PCs. You then have to account for these attributes for all of the monsters. Which is probably why Gary went with a simple mechanic in the first place.
What 4e got right
Yes, my fellow grognards, there were some advances in 4th edition, the biggest one in my mind being the rules for passive perception and insight. The passive mechanic allows PCs to spot secret doors (in elf-like fashion) without looking for them, and also provides a target for opposing stealth rolls to determine if PCs are surprised. Passive insight functions like 3.x's Sense Motive skill. Once you decide that you need a skill system, you need a way to keep it manageable, and passive skills fit the bill. Passive perception, in particular, helps deal with the trap-finding problem in prior editions, where the paranoid thief/rogue character searches every 10' square with a fine-tooth comb. Of course, that's what wandering monsters are for, too.
I also like the concept of the trained/untrained skill in 4e, though I'm not a fan of the execution. Trained skills in 4e are essentially ability checks against a set DC, delivering a flat +5 bonus for training, and all skills go up with character level automatically. I think this could be adapted to basic with tiers of bonuses for each rank of training (which is very similar to the Adventures Dark and Deep implementation).
How Skills are "bought"
If you have a skill system, you then have to decide how they are acquired. The Rules Cyclopedia uses a weapon proficiency analogue called skill slots, and allows the expenditure of multiple slots to simulate greater expertise. 3.x uses skill points and is by far the most customizable system. 4th edition essentially uses skill slots.
Adventures Dark and Deep uses an interesting xp purchase option, trading off xp for skills with a maximum number of ranks per level. I am lukewarm on this last option, simply because it is such a giant tradeoff for a player to learn an interesting "flavor" skill (like a social interaction skill) at the cost of hard-won xp. This system feels more like a GURPS-like, character-point driven game where character advancement is piecemeal (do you want to increase your skills or other abilities?) rather than level-based. I like point-buy games, having played Champions for years, but to me this runs counter to the level mechanic in all versions of D&D and I'm hard pressed to see where a player would want to learn any skills until he was of a fairly high level and the xp costs involved were proportionately less painful.
At the moment, I'm leaning heavily towards the Rules Cyclopedia as a way to provide some structure and specialization to skills in pre-3.x without getting overloaded in details or having to create monster skills to oppose PC skills. I also think having some kind of rules makes it more clear to the players how actions will be adjudicated. But I would love to hear how you deal with skills in your games.