Everything about the Forest Temple adds to its overall theme, and I think it does it very well. The Forest Temple has always been one of my favorite dungeons in Ocarina of Time, along with the Spirit Temple and Water Temple. I might go as far to say it's my favorite of my three favorites, with the Spirit Temple second and the Water Temple third. But I like the Shadow and Fire Temples as well...
Anyway, a lot of things add up to the Forest Temple's overall theme:
1. The music. It is strangely eerie and enchanting.
2. The environment. It resembles more of a ruined, haunted mansion than a temple or place of worship. There are outside courtyards filled with Big Deku Babas, Skulltulas, and Octoroks as well as interior rooms with paintings and symbols of the doors.
3. The enemies. While not all of the enemies are foresty, they all fit very well into the dungeon's "eerie" feeling. For example, there are Big Deku Babas, Skulltulas, and Wolfos for "foresty" enemies. For eerie or spooky enemies, there are Stalfos, Bubbles, Poes who hide in Paintings, Wallmasters, and a Floormaster.
4. The boss. While he's not a Foresty boss, he fits PERFECTLY into the theme of Ocarina's Forest Temple and is just perfect for the dungeon. And the room in which you fight him is also extremely fitting for the Forest Temple, especially because of the spooky paintings.
And that is why I love the Forest Temple.