If I'm running a pseudo-medieval fantasy game that doesn't have any explicit rules about horse riding, and a player with the appropriate gear wants to ride an available domesticated horse, I'm inclined to just allow them to ride the horse successfully unless some specific obstacle crops up, in which case I would either disallow it by fiat (in the case of an overwhelming obstacle), or establish some random odds of success based on a combination of the player's individual approach to the problem and whatever preexisting rules seem applicable by way of analogy (in the case of a hindrance that seems significant but surmountable).
This all depends on the assumptions baked into the setting; if there's a good chance your character has never seen or even heard of a horse before, I'm going to be more hesitant about letting you ride one automatically. But like I said, we're assuming a typical D&D-style fantasy setting in this example.
Let's say our gaming group switches mid-campaign to a different game, one that includes a horse-riding skill, and it says in the rulebook that only characters with this skill can ride horses. Absent any house rules, suddenly the ability to ride a horse goes from something players can take for granted to a specialized activity forbidden to all but the select few with ranks in horse-riding recorded on their character sheets, presumably at the cost of some resource like skill points that could have been spent elsewhere. The focus of the campaign shifts because the source of the conflict shifts; the set of all possible obstacles has been expanded to include the potential inability to ride a horse.
Because the thing about skills in RPGS is, generally no one gets all the skills, and everybody is limited in some area of expertise, because otherwise there'd be no need for the skill system. Why waste space in your rulebook and the precious time of your readers with banal, common-sense stuff like "all player characters are assumed by default to be able to wipe their own asses"? Why make players roll dice every time they want to take a breath? The skill system is presumably there because the game designers wanted to focus the action on certain things - "this game should be about killing monsters and taking their stuff" or "this game should give players the ability to make very different kinds of highly detailed and specialized characters" or whatever.
This is all well and good, as far as I'm concerned...unless it introduces limitations to the capabilities of characters that seem artificial or overly-constraining, to the point of either damaging verisimilitude, or just making simple actions into complicated ordeals involving too much time and thought. Much ink, both literal and digital, has been spilled over the introduction of the Thief class to OD&D, and thus the introduction of a class-specific skill list that left some people asking things like "does this mean my Fighting Man can no longer sneak around because he's not a Thief?" and "why can't my highly intelligent wizard learn how to pick a simple lock?" After all, soldiers in real-life are often trained not only in combat, but in skills that a D&D nerd like me is bound to associate with thieves and rangers and possibly other character classes. I've discussed this subject before. I'm personally undecided on the whole Thief matter - I'm probably leaning slightly toward the pro-Thief side, honestly - but I think it nicely illustrates the kinds of complications that a "skill system" can introduce to an RPG. Who gets what skills? How skilled does a given individual get to be? What does your skill list imply about your game's setting? How are skills linked to other mechanical aspects of the system, like class or level?
Originally, I wanted to make a list of some common, important skills in which I generally assume all PCs are competent when I run D&D and similar games. No matter your level, your ability scores, or your class, if you're a PC in, say my current LotFP campaign, you know how to do these things, and you can usually do them well enough that I won't make you roll dice to do them under normal circumstances...Except that I'm uncertain about many of these. So instead, maybe we should think of this not as a definite list, but as fuel for an open debate about what players should and should not be able to take for granted in an OSR game?
Anyway, consider this my incomplete, tentative list:
- Navigating and operating in the dark. Remembering positions and layouts from brief glimpses. Finding the right key or piece of equipment on your person in the dark without a bunch of fumbling.
- Mapping. But do cartography skills equal those of the player, i.e. should you make the players draw their own map?
- Estimating distances and measuring things.
- Good memory in general? If the actual players can remember it, then their characters can presumably remember it too, right? But what about stuff the players had to write down to remember? I would just assume their characters either have better memories than they do (thus not needing to write it down in-universe), or that they also wrote it down in-universe, as some sort of unspoken agreement between players and DM?
- Riding/driving/using horses, draft/pack animals, carts/wagons, etc.
- Feeding and caring for the aforementioned animals
- Building barriers? Basic carpentry? Probably basic tool use, at least.
- Fighting with many different types of weapons and armor (depending on the game and/or your class). Punching/hand-to-hand fighting without hurting yourself.
- Basic equipment maintenance
- Basic wilderness survival. Sense of direction, sense of time. Lighting fires quickly and efficiently and keeping them burning. Basic cooking, skinning/field-dressing animals? Hunting? Recognizing types of plants, at least in broad categories?
- Using rope, climbing ropes, tying knots.
- Carrying/hauling stuff without hurting yourself. Packing your equipment.
- Reading/writing! This is a big one, since in reality, literacy rates were extremely low until pretty recently, but in most D&D settings it seems like even the poorest peasants are fairly well educated in this regard.
- Basic math!
- Swimming. (Or at least floating/treading water?)
- Appraising treasure.
- Basic self-care/hygiene.
- Sleeping at will, often in shifts and in less-than-comfortable places. Sometimes even in armor?!
- Common manners/etiquette in the setting. One can choose not to employ it, but I'll assume you at least know how normal people generally behave in public.
For example, the Playtest Document for LotFP added a Seamanship skill, but in the current rules, I think it's implied that all PCs generally know how to sail, and success is mostly a matter of one's type of vessel and the size of one's crew, plus luck and maybe some other factors. Do OD&D players automatically know how to operate a ship? How about in AD&D?
Probably Unnecessary Afterword
Lengthy skill lists in RPGs are not a new phenomenon; I love the rulebook for Warhammer Fantasy Roleplay 1E, but speaking as someone who's only looked at the rules and not actually played yet, I'm kind of...I don't know, perturbed? Let's say, nonplussed by the game's lengthy, thorough skill list, which includes a lot of capabilities I would have thought to be common among the populace of the Old World (and hence the kind of things the players and GM could take for granted) combined with what seems to be a relatively low number of starting skills for most players.
I'm betting that in WFRP in particular, this probably isn't as bad as it seems, considering how many people vouch for the game's system to this day. Plus, my understanding is that a certain amount of comical ineptitude is purposefully baked into the intended playing experience, so I suppose it makes sense to start PCs off with few skills in order to embody the experience of playing bumbling misfits, at least at first. Plus a lot of the game's skills seem to be designed less as prerequisites to being able to do certain things at all, and more as bonuses (or the eliminations of penalties) to certain categories of actions - so presumably the OSR ideal of "letting you try anything" - allowing players to try and do anything in-game they could try to do in a similar real life situation - remains intact. Still, WFRP is the game that most directly inspired this post, so I used it as an example. Feel free to let me know in the comments how unforgivable of a sin this is.
What bugs me in particular is when a skill list is extremely granular, to the point of separating out skills that could easily be lumped together. It strikes me as nitpicky, overly concerned with a level of detail that isn't fruitful for creating fun gameplay. It clutters up character sheets and makes character creation and advancement more complicated, and can make one feel obligated to spend "skill points" or what-have-you on stuff that may be necessary, but isn't cool or fun. I prefer "stealth" over "hide" and "move silently", "perception" over "spot" and "listen", "athletics" or "acrobatics" over "balance" and "jump" and "climb" and "tumble".