Celebrim
Legend
For the thread in general I think the key thing to keep in mind when adjudicating these kind of actions is whether you want to encourage this sort of swashbuckling or discourage it.
Again, I play 3e and exactly what would work as good math in 5e for something like this I don't have the system mastery to say. I'm inclined to think that disadvantage for longer than one round would be too much though, since disadvantage is a worse penalty than dazzled. One thing you really have to watch is your action economy: that is, "How many actions are you stealing at the cost of a single action?" The answer for my ruling turns out to be, "Less than half of an action.", and it's highly situational, so I'm sure it is balanced.
But what I'm mostly struck by in this thread is how often on the EnWorld boards someone has felt the need to disparage my character and accuse me of being a DM that abuses his players because in there opinion I was the sort of DM that, for example, wouldn't let martial characters have any of the good stuff which I was supposedly unfairly and unthinkingly hording for the spellcasters.
And yet, now I'm being accused of giving away a far too generous martial technique without having thought through the consequences, in a system where most people probably are allowing this monstrosity:
Which by comparison has an area of effect, a range equivalent to a full missile weapon, a higher DC to save against, a longer duration, doesn't have implied restrictions on targets, and has a better secondary effect which allows no save.
If you start asking questions like, "How many actions are you stealing with the action of casting glitterdust?", the answer turns out to be potentially very large indeed, and certainly on average more than one. The same is true of something like a Tanglefoot bag by RAW. So in general, I tend to believe that my rulings are more balanced and thoughtful than the RAW, and that if you were to introduce them into unmodified 3.X that power gamers would find better things to do with their actions most of the time.