It's been quite some time since I posted anything to the conclave, but after reading through this entire topic, I thought this would be a good time to break my long-established silence and actually post something.
With regards to "reactions," I did a little experimenting with a friend about a week ago and came up with something that we think is quite a fun and dynamic way to represent reactions.
Ynek's suggested reactions rule:A character gets a number of 'reactions' equal to their speed, divided by three, rounding numbers down. Therefore, a speed 6 character would get two reactions per turn whilst a speed 2 character would get none (*1).
With GM discretion, a reaction can only be used to do something to react to something that the opposing character has done. For example, if a character is fired upon, it's reasonable to assume that they might try to shoot back or dive for cover. However, it would be a bit of a stretch for a character to "react" to being fired upon by continuing to hack a door code or continue summoning daemonic assistance.
When a character reacts, the reaction is resolved as an opposed initiative test with the character that they are reacting to. (*2) If the reacting character passes his initiative test, then he or she successfully reacts. However, if they pass their initiative test by a greater margin of success than the character that they are reacting to, then they react FIRST. This means that a high initiative character has a chance of diving for cover before the first shot is fired, or drawing their sword BEFORE the enemy tries to strike them. If the reacting character fails his initiative test, then this means that they fail to react and may not attempt to react again until the next action at the earliest. If the "acting" character that the reacting character is reacting to fails their initiative test, then obviously, the reacting character manages to make their reaction BEFORE the acting character makes their 'action'. However, there is no further negative consequence for the acting character. They still get their action whether or not they pass their initiative test because, after all, this is their turn and they've already passed their action roll.
So, in practice, this rule might work like this:
Example one - an example of a 'defensive' reaction.Character A spends his turn walking down an alleyway, minding his own business.
In Character B's turn, character B steps out from behind a dumpster and fires a shot at character A. Character A chooses to react to dive into cover.
Both players roll a D100.
Character A has an initiative of 80, whilst character B has an initiative of 75. Character A rolls 55. According to the "roll high, but under," convention described by Marco earlier in this topic, Character A has passed their test by 55. Character B rolls their initiative test and rolls 65, passing his test by 65 points. A glance at the two D100s will reveal that 65 is higher than 55, and both characters have rolled beneath their initiative values. The outcome of this dice result is that Character B fires their round at character A, and immediately after, before any following actions by Character B, Character A dives into cover.
Example two - an example of an 'aggressive' reaction.Character A has been followed back to his lair. He closes the door behind him, draws his gun, and aims at the doorway.
Character B follows him and opens the door. The GM rules that an awarness check is required to notice the 'trap' that has been laid, with an appropriate modifier depending on how obvious the trap seems to be. Character B fails the awareness test and walks right into the trap.
Character A chooses to 'react' to the opening of the door by opening fire with his gun.
In the opposed initiative test, Character A rolls a 95, whilst character B rolls a 50. Character A, the reacting character, has failed his initiative test, and therefore does not react when character B opens the door. Character B opens the door and continues with their remaining actions (A smart player would have chosen to make a pause for breath after opening the door... Because the character doesn't know what might be behind it, and it makes sense that a character would pause for breath in that moment.)
An alternative version of this rule that I'm tinkering with alongside my usual gaming buddy is that a character is limited to one SUCCESSFUL reaction for every three points of speed. This means that if you fail your reaction test in this action, you can try again after the next. It seems to slow gameplay down a bit, but it does mean that just because a character fails to react in the first instant that a change occurs, they're not left standing there getting shot at for the several actions that follow in the remainder of the turn. Bearing in mind that there might be three or four enemy characters who get their actions before he does, it doesn't really make sense that just because he failed to jump for cover after the first shot was fired, that he just decides to stand and get blasted by every gun in the enemy's arsenal.
Of course, it's not a perfect rule... But I'm humbly submitting it for your consideration.

(*1)= Such characters are likely to be too slow to react in time to any sudden changes in the situation. However, considering that Speed 2 is usually reserved for Orks, Ogryns, servitors, cripples and the like, I think it's safe to say that they aren't exactly the most reactive of characters anyway. Similarly, the only characters which are speed 6 are usually hyper-elite assassins and aliens such as Eldar, who are almost certainly going to be very reactive and fast to respond to any changes on the tabletop.
(*2)= This means that a slow and dull-witted character is easier to react to than a lightning fast one, and this is something that any rules for reactions ought to consider. For instance, it's easier to dive for cover and make it in time when your opponent is a dull-witted servitor with slow response times than it is to make the same dive when you're trying to avoid getting hit by a highly trained Eldar assassin.