Because most of my group have done the Challenge modes before, they knew what to expect, which helps both with strategy and where/when to move - getting Silver is a given.
For my part, I've had to learn exactly where to use my cooldowns, potions, CC, everything, because if I do something at the wrong time, a few seconds is the difference between success or failure.
So far, my experience has been thus:
- Siege of Niuzao Temple we 1-shot Gold almost by accident while cruising for a Silver.
- Temple of the Jade Serpent looks to be another easy one, since we got close to Gold without trying but haven't been back yet.
- We took a few runs to get used to Stormstout Brewery and another few resets for bad RNG to get Gold, so I'd call that the average.
- Scarlet Halls trash packs are overwhelming for our group, so that one ranks hardest so far.
- The Silver run of Mogu'shan Palace I can see being somewhere between average and hard, since we had a few deaths, but since I've only been there once it's a little hard to tell whether it will be harder than Scarlet Halls or not.
- I haven't tried the other 4, so can't comment on them just yet.
All in all, Challenge modes are a lot easier than I thought they would be. Then again, I have great guildies, so perhaps I shouldn't be surprised!