Very tired of the Atma-sphere issue here so I will contribute to the theme of this thread.
Major disappointments for me? Thiel 3.6 and 2.3 speakers!!! These were way too analytical to me. Replacing these with Talon Khorus and Talon Peregrine were major improvements across the board. Sadly the pricing game with Talons gave the company a bad reputation. But these speakers are awesome nevertheless for both music and HT systems.
Another was the Audio Research PH3. Way overrated. The PH2 that I had been using for 10 years was vastly more smooth and tonally coherent than the bright and severely colored PH3.
Oh and finally, the Audioquest Diamond ICs. Wow, what a horribly dimensionally flat cable this was! I admit to using the highly tonally colored NBS Statement ICs at the time, but at least these provided the spatial features within the music.