New to this Forum, first post. This thread seemed interesting so I felt I'd respond. Note I didn't read any of the other comments before writing mine, so mine may overlap with some of the previous posts.
My most underwhelming classical book(s) has got to be the J.R.R. Tolkien's the Lord of the Rings. While the work itself may have pioneered classical fantasy and all the novels, movies and tabletop games that came from it, actually reading it was quite a chore. The depth of the world portrayed within the book and the vibrant mythology that intertwines the epic historical events of the First and Second Ages, and the sheer effort that must have gone in creating several complete fictional languages and their corresponding alphabets belies the quality of the storytelling in the book itself, which in certain places reads more like a stiff documentary than a flowing, fictional story. As a result, the Lord of the Rings was, to me, an underwhelming read that serves to starkly underline the differences between a linguist and a writer. The first is needed to make the LEGOs, and the second uses them to build something great. Tolkien just made his LEGOs and then left them in the box.
My most underwhelming movie classic has got to be... hmm, this is hard. I've seen thousands of movies in my life, but if the words of internet critics are to be believed the recent film Inception is considered a modern classic, so let's go with ... Inception. From the critiques we have read we were promised a revolution in filmmaking unlike any other, creating a movie that couldn't be categorized about layered dreams within dreams within dreams until you didn't know you were still dreaming. Roger Ebert even claimed that the movie was 'immune to spoilers'. I enjoyed this movie a lot but I couldn't help feeling cheated somehow. While the visual effects may be called astounding with a straight face, in the end the elements used to visualize the mental conflict inside of a dream were the same old staples of regular action movies; gun fights and vehicle chases. Wow. Movies like eXistenZ and The Cell did a better job of illustrating the contrast between reality and dreams, and they did it with a budget much smaller, both in terms of money and time. Way to turn a screen play ten years in the making into a somewhat above average action flick rife with pseudo-intellectual babble, Mr. Nolan. I recommend checking out Paprika instead, an anime cited by Nolan to be one of his inspirations for his magnum opus, and infinitely more interesting.
My most underwhelming video game classic has got to be the first Halo (or, alternatively, the entire franchise). The X-Box needed a good launch title at the time and they ended up choosing a game about as colorful. It was a good marketing maneuver though; look at all the sequels it's got now. I never played it on the X-Box, but I finished a run through on the PC. I kept waiting for the immersion factor to set in, for some switch to flip in game to suddenly make me interested in what happened and what was going to happen, to make me feel like I was into it. But it never came, and it's not just the overuse of gunmetal gray in the game. The original 1997 (or was it '98) Half-Life had plenty simplistic areas in the game and yet it managed to suck me in easily, and kept me glued to my monitor until the very end. Anyway, Finished Halo once, and never touched it or wanted to touch it again. A true classic should always leave you wanting more, but this didn't.
My runner-up most underwhelming video game classic is the single player campaign of the first Neverwinter Nights. I love Dungeons and Dragons and I love video games, but this classic game's horrible, horrible pacing was like a dagger in its back. It's got quite a vibrant multiplayer community, though...
What saved NWN's single player from this list for me were the add-on campaigns; Shadows of Undrentide and Hordes of the Underdark which were infinitely more engrossing.