I think the thing I like about Timberline is the feeling of transition down from snowy mountains into green, mountain stream areas below the "treeline".
Speaking of snowy, Frostgorge is one of my favorite zones in the game. I love the Kodan ship/icebergs and those deep, underwater arctic views.
Hunting balloons for the acheivement has been interesting. I think it's the first time I've ever moved through so many zones in the game so quickly, which is actually really enjoyable. It gives me a new sense of the breadth of the world.
I'm also very fond of some of the areas in the game that have a lot less "content" than you'd expect for how interesting they are. I was blown away the first time I stumbled in to Wizard's Fief. It was early enough in the game that I hadn't heard about Wizard's Tower being there. There's a whole little mountain hamlet beneath it as well, with a skill point, a vista, diving goggles, and some small defeat-an-elemental DEs. While the tower is intriguing, and makes me want to be able to explore it, I'm kinda glad that there isn't more to do there so the area isn't overrun by farmers.
Lightfoot Passage is another one. It's kinda overshadowed by Fort Trinity and the beginning of Orr. Just a Vista inside. When I first wandered in it was a delight to find this secluded-seeming preserve right at the doorstep of Orr.
I know this has wandered off the original post, but I'm now curious if anyone else has a favorite, less-traveled corner of Tyria.