This hasn't received much discussion that I've noticed, but a very nice little quality-of-life improvement has been made in Mists of Pandaria: stronger snares no longer wipe weaker snares off the target.
Ever since vanilla, the normal behavior of snares has been that a stronger snare removes a weaker snare from the target when applied, regardless of duration. I recall a chat with an in-game support rep one day while I was farming Guardian Stones in Un'goro Crater; I was annoyed that if I had a Cone of Cold snare on the target, the Frostbolt snare would not apply, and thus the mob would start running full-speed when Cone of Cold wore off until I hit it with the next Frostbolt. Ultimately, we determined that this was intended, though irritating, behavior.
Some spells, such as Slow, would not be wiped, because their non-snare components could not be overwritten by a snare. But most of us Frost Mages became so accustomed to the overwriting behavior that we didn't even think about it anymore.
But someone did think about it. A fellow named Halfawake posted back in May to complain about this behavior. He was right — an ability like Frost Bomb, with its two-second 70% snare, is a lot less appealing if it's wiping a six-second 60% snare off the target. And a few days later, Ghostcrawler responded to inform us that the behavior would change.
The Current Design
Let's look at a scenario: say you hit a target eight yards away with a Cone of Cold, and then blink away (with a glyph). You're now 33 yards away from the target, and it's got a 60% snare on it for another 4.5 seconds. For the sake of making this simple, we'll assume you have no haste.
At normal run speed, the target would take about 4.7 seconds to cover the 33 yards to reach you.
If you do nothing and just let the Cone of Cold tick down, the target will cover 12.6 yards in the next 4.5 seconds, and then take 2.9 seconds to cover the remaining ground at full speed, reaching you in about 7.4 seconds.
But now let's imagine a Frost Bomb detonates on the target two seconds after you cast Cone of Cold. What you get now is a half-second of 40% movement (1.4 yards), two seconds of 30% movement (4.2 yards), and then 3.9 seconds of full-speed movement to cover the remaning ground, for a total of 6.4 seconds. Applying the stronger snare actually reduced the travel time by a full second.
Now of course you're not just going to stand there; you'll cast Frostbolts. But even so, you're almost certain to wind up with windows of full-speed movement; if a Frostbolt hits the target a half-second before the Frost Bomb snare expires, the target's going to move at full speed for a second and a half while you finish casting the next Frostbolt and it travels to the target.
The New Design
Under the new design, the effects of snares don't stack, but the debuffs themselves do. When a stronger snare expires, if a weaker snare is still on the target, it takes over. So revisiting our scenario above, the target now moves for a half second at 40% speed (1.4 yards), then two seconds at 30% speed (4.2 yards), and then another two seconds at 40% speed (5.6 yards) before returning to full speed to cover the remaining distance. Total travel time is now 7.6 seconds.
What all this means is that stronger snares like Frost Bomb and Cone of Cold will now work with, rather than against, weaker snares like Frostbolt, Frostfire Bolt and Blizzard; applying a stronger snare, even if only briefly, will always reduce the target's average speed rather than increasing it.
In practice, simply running around questing, the change is subtle but feels very good. It makes more sense and it feels more natural. It's not a big thing, but it fixes a longstanding problem that most of us had become so accustomed to, it never even occurred to us to ask for it to be changed.