Although this might change in the final version, here's how the skills system is turning out now.
Skills are broken down into levels (and, in some cases, tiers, with each tier having a full set of levels). Each tier/skill level has a standard DC (difficulty rating), and a standard MP Cost (used to determine how many skills you use in combat). The abilities listed for each level will have that DC and that MP cost, always. Similarly, the XP costs to train a given level do not change.
In general, all the abilities that are listed for a skill will be variations on the same. The main difference is in the special bonuses attached to each level; at the lower levels, they come with hindrances, preventing you from using them effectively, while at the highest level, they offer special abilities.
There are four skill levels, numbered 0-3. Zero level is "Untrained"; first level is "Novice". Second level is "Competent"; at this level, you can expect to start getting skills that are not hindered or which have special effects. Third level contains mastery skills, which will tend to be specialized; either the skill itself will have specialty abilities at 3rd level, or you may select special abilities from a list, which are permanent to your character.
Certain skills are themselves a specialized version of another skill. For example, Martial Arts is a specialized version of Weapon Proficiency. These skills have prerequisites; in order to gain a level in Martial Arts, you must have an equal or greater level in Weapon Proficiency. In return, you may find yourself using abilities at a lower level than you did previously; Novice-level martial arts abilities may be equivalent to Competent-level weapon abilities, for example. Because the DC is lower, you will get more critical hit bonuses on the Martial Arts ability, which is especially helpful when taking multiple actions per round.
When you take multiple actions per round, you are said to be using a technique or "Tech". There are special rules that apply to techs.
First, you can only roll one skill roll per round; every other skill roll takes its AAR value. AAR stands for "All Average Rolls"; Every D4 is counted as 2, every D6 as 3, D8 as 4, and D12 as 6. For commonly used skills, you may want to write this number down for reference. Naturally, if your AAR is not high enough, you cannot perform the skill this way. This is especially important to remember if you take attribute damage, as it can substantially limit your options.
Second, any failed skill (Whether rolled or performed at AAR) ends your round immediately. You retain any unspent MP for defense actions, although the MP you spent on the failed skill is spent normally. Additionally, any ability that has the Turn Eater hindrance (usually seen on 0- and 1st-level abilities) will end your turn immediately, preventing you from using any other actions, including mental and social actions.
Third, if you gain critical success bonuses on any skill, you can refuse to apply them to that skill. If you do, they pass on to the next skill you apply during that round. If you do this, you can chain several skills together, producing a final attack with a stack of additional bonuses. This is especially useful for allowing you to specialize in other skills (Jump, run, acrobatics, perception, aim, etc) and apply these to relevant situations.
There are of course a couple caveats. If you use an ability with the hindrance Crit Eater (Almost all 0-level abilities have it), any critical successes you had previously disappear; they cannot be applied to that ability nor any that follow. Additionally, the GM may rule that certain skills are incompatible, meaning that you cannot use one to gain bonuses for the other. For example, critical success at lockpicking will not allow you to land strikes in a melee, nor will sword proficiency help at picking locks. In addition, any skill that takes more than a round to perform cannot be used as part of a tech; you cannot perform other actions leading up to the start of a long skill, nor chain other actions after its end.
Fourth, there are certain situations in which multiple people can contribute to one Tech. This is prevalent when discussing magic, but also works for knowledge skills, group combat, crafting, searching, and other cooperative tasks. Not all skills will qualify, and the above restrictions apply, can use long skills as long as they're being performed concurrently, or in some preset pattern. For example, multiple crafters of different kinds may work on a project in succession, rather than at the same time. You'll have to ask the GM what sort of cooperation they'll allow.
Fifth, you will notice that certain weapons and skills have a "Crit pool" (term subject to change) that contains unique or rare kinds of bonuses. For example, long or chain-type weapons might have the "Trip" bonus in their crit pool, while an assassin's dagger may have the "Stamina Attribute Damage" bonus. When using that skill or weapon, and ONLY when using it, you may opt to transform a normal critical hit into one of these specials. If you inherited bonuses from earlier in the round, you may use them to select from the crit pool; however, the crit pool is not passed down to the next skill in the line. Unless otherwise noted, special crits do not stack (can only be successfully used once per round, and cannot be used more than once per ability), and you must successfully perform the ability (land the attack, etc) for the crit to take effect.
If the enemy has armor or a magic effect that blocks that specific type of critical, the armor automatically wins, even if the opponent does not take a defense action; however, the armor takes damage from the blow, and so has a limited number of uses. If you notice the armor break when selecting such a crit, the next time you use it, it will likely get through.
Note that the tech system is extremely powerful, and also extremely flexible. It allows characters to specialize in various skills and apply them to related situations; however, it also exposes the vulnerabilities of a given fighting style. If most of your skills revolve around a particular attribute (Strength, perception, etc), you are far more vulnerable to attribute damage; if you rely on some particular skill or item a lot (A sword, the running skill, etc), enemies can adjust the situation to prevent you from using them (Disarming you, spreading caltrops or other hindrances, etc). If you wear armor or make other notable changes to defend your weaknesses, you risk tipping off a well-informed enemy as to what sort of skills you excel at. However, if your style is spread among many skills, you'll have less experience in each of them.
The tech system is also useful for GMs. Since the DSP settings (as currently envisioned) use humans as much or more than monsters, there has to be a way to mix things up without giving the GM extra work. However, the "Monster Manual" concept from the D20 and similar systems gives the players an even chance of having out-of-character knowledge of the fighting tactics and weaknesses of an enemy. For the DSP, therefore, a good GM can choose to create a generic fighting style that a group of opponents use; for example, enemies that use blinding attacks, tripping weapons, martial arts, and swimming skills to drown opponents.
The GM can have any number of mooks (lesser random encounters) that use that style, but facing them will be entirely different than fighting mooks of another style, meaning that they are essentially different monster types. And until the players first see or hear of this fighting style, they can't make any preparation, but once they do, they can start devising countermeasures, and certain characters may choose to specialize in fighting against that style. If that style of mooks becomes a major part of the story, those that specialized in fighting that style become major players themselves.
On the other hand, players that have a glaring weakness in their fighting style and then become famous will find that enemies quickly adapt; they may find that enemies start having armor that blocks their specials, and weapons or skills that overcome their weaknesses. At times like that, teamwork is particularly important, as long as the entire party doesn't have the same weaknesses.
All in all, I really like how it's turning out. There's some drudge work I need to do before it's ready for a playtest (filling out skill lists, equipment lists, bonus lists, etc), but I'd like to see it--and I'd like to PLAY it. It seems like it'll be a lot of fun. I hope you'll like it too!