For a minute I thought you were referring to the Forest Temple from Ocarina of Time , which is my favourite dungeon...
Even though I don't hate the Forest Temple from Twilight Princess I still see it as a rather basic dungeon, now this dungeon tries to escape from the tutorial based dungeon that the likes of Inside the Deku Tree from OoT. However the fact is that TP had a large build up, before you got to the first dungeon you probably went from anywhere to 1-4 hours maybe even more for some. So before you enter this dungeon you get a pretty good picture of the game, the controls, the feel etc. all of which feed you into the experience.
So considering the large build up to the first dungeon then, it didn't really match it with a true challenge, this is where I think the dungeon could have improved. It wasn't really the enemies that lacked true challenge but more so the puzzles. There were a lot of lighting torches, which is probably the most basic puzzle in the series and a puzzle that I think should be, either made more advanced or be left to tutorials. Then you had the Gale Boomerang puzzles which, were again, quite easy. There was also the problem with the boss/mini boss, now this was evident throughout most of the game but I think the first dungeon should really give us a run for our money, with only three hearts the first boss should really threaten Link however both bosses, in this case, didn't.
So challenge was probably my only gripe with this dungeon, the size of the dungeon and the general content was perfect for a beginning dungeon and it started off a chain of large and expansive dungeons within the game. The forest theme was also more evident in this dungeon than it was any other to date, it just had that feel like you were trapped within a deep forest surrounded by bugs and wild plants. I also rather enjoyed the layout of the dungeon, having to navigate round by saving Monkeys was a unique system and it was different than "find the key" that we see a lot of the time. One more thing is would like to mention is the brief outside sections, these "outside" areas really add to the dungeon as a whole even if they were very brief. It just gives that sense of scale and differs from the usual claustrophobic atmosphere.
Overall it was one of the weaker dungeons in Twilight Princess however, comparing it with other starters it was certainly a step in the right direction, my only true criticism would be the challenge.