Well, every zone has a (usually well-hidden) puzzle which awards a bag of level-scaled loot. The Lake of Solace has the level capped at 30, Stonefield is 35, and Scarlet Gorge is also 35. It goes all the way up to 50, eventually. Blue quality stuff is the most common, but purple and green is also possible, for better or worse.
The reason that Defiant also get more loot is because there are Cairns, which give the exact same quality stuff but without requiring any puzzle-solving skills. The first two Guardian zones have none of these, while the Lake of Solace has eight, and Stonefield has another. Of course, there's nothing keeping Guardians of any level out of the Defiant zones, so the only real disadvantage is accessibility. If you know about the puzzles, you'll get them all fairly easily.
On that note, if you're not afraid of sneaking your way into Guardian territory, the Gloamwood puzzle is located
here (note that the topmost Cairn has a level cap of 40 and technically counts as part of the other zone), and since it's hands-down the hardest puzzle in the game,
here is a solver. Silverwood's puzzle is much, much easier, and you can find a plaque on the wall which starts it at (6415, 3154).
There's plenty more, too. It's great for gearing yourself out, although some of the loot can be disappointingly bad.