Glare is one problem, but I find seeing my reflection in the screen extremely distracting. Something about that human face keeps pulling me out of the "zone" while I try to code.
PS: I don't know of a better word than "zone" so I hereby name that state Zem, it's like Meditation with the added creativity of Rem sleep. Feel free to start using it in everyday speech.
I plug mine into an external keyboard and mouse at home and at work, but still use the laptop's keyboard and trackpad out of habit most of the time. I think I gravitate to the consistent interface of the laptop rather than the differing setups at home and work. There must be some human interface research on this. That or I am weird.
Glossy screens are a killer if you like working in areas with lots of natural light or sunlight.
Try wearing a white tshirt, sitting in partial sunlight, and using a glossy LCD>
Having said that, I find that smaller screens are fine with glossy - like my Acer Aspire One netbook. The bigger the screen, the more of a problem it is.
Perhaps it affects people who prefer a light on dark color scheme as opposed to dark on light. Glossy doesn't bother me at all, but there is a lot of white on my screen.