Well, from my Stargate RPG review: (Note that I believe regular Spycraft has fluid initiative as an option, not the default)
"As mentioned, combat is fairly different. Simplified in some areas, more complicated in others, like initiative.
Gone are "Attacks of Opportunity" (not surprising, as many d20 variations seem to do away with it, at least ones featuring ranged weapons as the most common sort of weapon)
Also gone are "Full Attacks", in which high level characters could make multiple attacks per round. Instead, one attack takes a half-action. So a character can make two attacks per round if they want, or one attack and one other half action. (Somewhat similar to regular d20 in which you can attack once and move)
Initiative changes the most. While its basically the same at its core, you roll a d20, add modifiers to get your initiative score, and then in each round, each combatant moves according to the highest roll, the initiative score is now "fluid". That is, it can change, depending on the action the character takes. This takes some getting used to, and it takes some effort to keep track of, from round to round.
Armor is also handled somewhat different. Each class gets a "defense bonus" to armor class, based on level. But if they use armor, this bonus is forfeited. Armor instead provides damage reduction, and in some cases, a bonus to defense (but usually a small one).
For instance, the Kevlar Vest has a Damage Reduction of 4, but a Defensive Bonus of just +1. (Most armors don't have a defensive bonus to ac, and some heavier armors have negative ones).
Another interesting addition is "Action Dice". These are basically additional dice that the player (or gm) can use to improve any skill or combat roll. They are generally small dice (d4s), and most characters get 3 per session to spend (you can buy more using feats and such).
Besides helping improve rolls, action dice change combat dramatically, because they are integral into how critical hits are handled. While a critical "threat" works the same, critical hits happen only when a player uses an action die. This does add to the flavor of combat, but also adds more record keeping. The GM also gets action dice, to use against players whenever he wants, and also has to use it to score critical hits. "
