My personal take is that any time a player wants to do something cool, you can improvise a ruling for it using the stunt rules. By doing this, it means players can "break the rules" and do things that are not explicitly outlined in the existing powers, like leaping to grab a rope and use the swing to kick an enemy into a pit. In practice, this means I use a system of damage shifting that works roughly like this: I look at the player's action as an attack and decide where I'd rank it on the scale of
- Dull, repeatable
- Interesting, repeatable
- Awesome, repeatable
- Dull, Non-Repeatable
- Interesting, Non-repeatable
- Awesome, Non repeatable
Those numbers correspond to the columns for Damage Expressions on page 42, and for something that's straight damage, that's all I need, but what if it should do something else? Was the swing kick supposed to push the target back? Was the burning oil supposed to ignite?
In a situation like that, I just slide down the list based on how potent the effect is. For simple stuff that I might see in an at-will, like pushing the target a square or letting the PC move a little farther, or a small amount of ongoign damage I'll go down one step. For more serious effects, I'll slide down a few more steps. Easy peasy.
But this is where it gets rough, and where we get into my biggest frustration with 4e. So long as we stick with things we can easily visualize, where the consequences of the actions are reasonable and understandable, this all works great. Imagining an action that knocks someone down or lights them on fire is not hard, but many other effects are much harder to visualize. The idea of stunting an attack that might put the Dominated or Petrified status on someone is hard to wrap your brain around.
But the problem is, that's equally true of powers. The color of 4e attacks varies from loosely tied to the mechanical expression to completely baffling in its association. This encourages a habit of expressing things purely in terms of mechanics rather than color because the color will, simply put, not always make sense. That, in turn, discourages things like stunting because it's an entirely different language. Put most directly, there are two modes of play - applying mechanics and then describing their outcome, or describing action and then interpreting it with mechanics. A lot of times when people decry 4e as just a boardgame, it's this divide they're talking about. I don't think that's a fair comparison, but awareness of this division is useful.
So, the problem with coming up with color to reflect more dramatic effects is a rough one, but this is where we also come back into 4e's strength. While it's not quite a science, there is definitely an art to when certain types of effects and conditions start occurring in play (both in terms of monster abilities and player abilities). Dominate, for example, it not terribly common at the heroic tier, but in higher level games you'll see more of it. This pattern is true for most effects, and this makes for a weird ebb and flow of play. On paper, these things aren't really level dependent, but by distributing them by level, 4E uses its mechanics to create is own type of flavor for the tiers of play.
The upshot of this is that as players and GMs achieve greater mastery of the system, they also get a greater understanding of what are, effectively, the genre expectations of the tiers. A heroic tier monk who attempted to justify a paralyzing strike as a stunt would probably be considered inappropriate, but they same monk in the epic tier might be more in bounds.
Now, genre is definitely a bit of a bugaboo. Let's say that monk wants to jam his hands into a burning brazier sacred to Kord to make an attack with his fists wreathed in divine fire. In some games, that would be totally cool, while in others, the monk would be looking to take some fire damage for his trouble. Neither approach is incorrect, it's only a problem if this expectation is not communicated well.
That said, I do caution any DM looking to get his stunt on to keep an eye on the roles of the party member and be more flexible when a stunt reflects that role. Similarly, this is probably the area where power source is well worth keeping in mind. See, Martial characters and Strikers (especially martial strikers) have the easiest time stunting. Because they're physical and all their actions are easy to describe, they're easy to stunt. Characters with other power sources are dependent on their powers to do things, and as powers are purely a function of game logic, it's really necessary to develop a sense of what cool things the character could do that are in theme but not necessarily power use. This is easier for some classes than others - Druids are MADE to stunt, but Wizards can have a hard time of it because of the historical D&D model that their abilities are all Vancian spells - so it may be worth talking to individual players about how they see their character stunting. Even if this means coming up with a few "pre-made" stunts and setting up situations where the character can use them, it is entirely worth the effort of doing so if it means EVERYONE at your table is engaged and being awesome, not just the rogue or monk.
1 - In combat, at least.
2 - In this I also include some faux conditions like "Unbalanced" (getting hit again knocks you prone) or "Distracted" (grant someone else combat advantage)
3 - There's an argument that this makes players more powerful because it effectively grants them an array of extra encounter powers so long as they can think of awesome things to do. I am too pro-awesome too give this much weight. Also, there are some checks baked into this model - I'm willing to have damage go to 0, representing a zero step on that list, but no lower. That means if a player wants to kick an opponent back one square (dull, repeatable, 1 shift), then he can do it if he's willing to forgo damage, but if he wants to trip an opponent (dull, repeatable, 2 shift) then it's not an option unless he does something to make it cooler. In practice, this means that players can't really make up encounter power-equivalent effects based solely on their own actions, though a creative enough player might be able to effectively fake some at-wills. However, each time you do something it gets more boring, so coming up with "Repeatable, Awesome" is great, but actually repeating it it will make it interesting, then dull in short order.
4 - Ironically, this actually creates a lot of overlap between 4e and some of the more curious small press games, which also flow from mechanics to play rather than having mechanics used to interpret play, but that's neither here nor there.
5 - In my mind this is a callback to the way spell availability changed the nature of D&D over time. Certain spells (Teleportation, Fly, stuff like that) opened up the scope of play by changing the nature of things like overland travel, which in turn changed the flavor of games. 4e powers don't really reflect this (though rituals do , to some extent) but the game is still structured to try to capture that sense.