I'd agree that you don't have to like the the HD-650 to like the LCD-2, but if you do like the HD-650, there's a good chance you'll love the LCD-2. The HD-650 has accurate detailed treble, but it's pulled back a bit--at least compared to bright signature headphones. It's clear that people have very different sensitivities to upper mid/treble energy. If you get too much, it comes across as harsh and often sibilant. If you're not getting enough, a headphone may seem like it's muddy or lacking detail or air.
It has everything to do with psychoacoustics--how you perceive sounds. My observation is that although there are a lot of fine-grained preferences when it comes to headphones, the tolerance to treble energy is the least negotiable aspect for the majority of people.