I downloaded the free version of X-Plorers a long time ago, but I never really gave it a serious read through. JB recently posted a glowing review for the game, so I gave the PDF another look-see.
While I'm not sure this game will ever supplant my undying love for Stars Without Number, I do like the way they handle skill- by baking them into the character class.
Every class in X-Plorers has a four pretty broad abilities that all members of the class possess. Skills are checked by rolling a d20 and trying to meet or beat the character's skill number. The roll is modified by the appropriate attribute bonus/penalty. (Ability score modifiers are understated, capping at +2/-2 at scores of 4/17) Skill numbers decrease (rather like saving throws) as a character advances in level. I should note that the Soldier class is an exception- of his four skills, two of them are actually combat related bonuses (number of attacks, unarmed damage, etc.) that simply improve as he advances in level; you do not roll to check these skills.
I really like the basic idea here, which is making skill mechanics nearly identical to saving throws. I also like that a character class has skills attached to it. I always liked the idea of character class implying the different areas of a character's expertise.
If I ported this into D&D, I would probably change a lot of things about the implementation while leaving the basic mechanic intact. I'm thinking that classes would have some skill choice- it bothers me slightly that every Soldier in X-Plorers has training in demolitions and every scientist is also a doctor. I see the potential here to create variant fighters, thieves, etc. without creating new character classes for everything. Imagine there is a class skill list, and you are allowed to take X many choices at character creation. You could take Tracking and Favored Enemy to create a "ranger", or maybe Tracking and Stealth, etc.
I imagine many players would want there to be a little bit of potential for "cross-class" skills. D20, of course, did it. Even AD&D 2nd edition allowed you to take NWPs that were outside your class group if you paid extra slots. I'm thinking of two ways to do this right off the bat:
1. Cross-class skills cost two skill choices, rather than just one. (Consider that in X-Plorers, each class has only four skills.)
2. Cross-class skills cost the same number of skill choices, but they improve at only half the rate of class skills. For X-Plorers saving throw-style skills, this means that cross-class skills improve only every other experience level, rather than at every level. It means that combat skills would only improve once every four levels. Hmm...that seems rather harsh. I'd be tempted to say every three levels.
Thieves, of course, would get the most skill choices, and their success rates would be converted to X+ on a d20 (with starting rates more analogous to the Scout class in X-Plorers) Perhaps they would receive 6 or 8 while the other classes received four.
This is where it could potentially expand beyond where I had intended it to go. Perhaps magic-users would only receive three skill choices, with the use of magic counting as one of their abilities. Would clerics deserve the same treatment? (Spells and turning...do they receive only two skill choices?) Does this mean that a class could take magic-use as a cross-class ability? You could create a Bard or a Grey Mouser style character by playing a Thief with magic-user spells as one of their skill choices.
I think I'd have to reign it in a bit there. If we're getting that abstract, why not remove character classes entirely? (Short answer: because I like character class as a part of D&D) I'm thinking that magic-use or other supernatural abilities might cost double and advance at half rate, while more mundane skills like Horsemanship or Legerdemain would cost normal and advance at half rate. After all, you can easily argue that learning how to cast magic spells is probably a smidgen harder than learning how to move quietly or gather food in the wilderness.