The main thing I've always heard is that you're supposed to avoid "negative" angles on your wrist when typing. So I have a very thin, flat keyboard that I normally use.
Laptops are a similar concept, but they are raised up enough that my normal typing posture doesn't work that well. With my Inspiron that I have at home, the base of the laptop is thin enough and angled decently that it works out fairly well.
The Thinkpad has a base that is almost an inch thick, and my hand is just the perfect size that the hard edge I referenced is exactly where my wrist bends, which is I think what makes things so uncomfortable when using it.