Scenarios are another one of those features that have been new in Mists of Pandaria and thus new to me. I haven't done all of them yet, but I think I've seen enough of them (plus a couple of heroic ones actually) to be able to form an opinion. Generally speaking, most of them seem to be pretty fun the first time around - for the sheer novelty value if nothing else - and then become dull as dishwater once you start repeating them. I've been trying to put my finger on why that is.
I think one of the major issues I have with them is that they just feel a bit... clunky. In terms of gameplay, scenarios seem to fall halfway between a regular quest and a dungeon, but it seems to me that the aspects they chose to incorporate from both sides don't really go well together. Like many quests, scenarios seem to like telling a story, but unless you just happened to find the intro quest to that specific scenario out in the world, you'll be dumped smack in the middle of things with no explanation of where you are, what's going on, or why you're doing what you're doing. In a dungeon, not knowing the background usually isn't as much of an issue, since they all share the overall "theme" of clearing an area of bad guys and defeating a big bad or two, plus the dungeon quests tend to at least give you a brief summary of the most key points. In a scenario you got that little box to track progress and that's it.
To add to the confusion, many scenarios seem to like to make things "interesting" by requiring lots of special actions from you and your character, such as collecting supplies, planting explosives, firing cannons, swinging from ropes etc. They are always marked quite clearly and big red text in the middle of the screen usually tells you what to do, but still... let's just say that expecting people to figure out new game mechanics while being thrown into a typical "rush rush" style WoW pug was not the smartest idea Blizzard's ever had.
On top of that, scenarios seem to be extremely unrewarding, which makes repeating them feel like a waste of time. If you queue for a random, you do get a small reward for that, but if you do a specific scenario you get absolutely nothing for it, nada (unless you count the satisfaction of actually getting the one you had a quest for). The bosses drop nothing, and you won't even get a couple of silver off the trash. I don't know why that is; it's a small thing but it really leaves me feeling disappointed after every scenario completion.
The sad thing is, for all these flaws I could see scenarios make for an interesting addition to the levelling game, just to do something slightly different from questing or running dungeons all the time. Just getting XP for each one would already make doing them a lot more worthwhile. But no, Blizzard restricted all of them to max level, where players will have a dozen other things to do that are more fun, more rewarding or both at the same time. It just strikes me as a lot of wasted potential.