Some diligent soul has put together a most comprehensive comparison of the differences between Lucky Star’s Tsukasa, as drawn by each animator of the series, as well as Haruhi’s characters.
Since anime is generally produced in parallel by multiple teams of animators, there are generally slight differences between each team’s output, which may be very noticeable if supervision is lax or the animation is rubbish – I am not afraid to admit that I cannot tell them apart at all; their animation supervision seems top notch, or perhaps I am not looking hard enough.
However, a look at the Haruhi animator comparison on the same site does reveal some differences, however minor.
As a tribute to this dedication, I reproduce the images below, should you wish to try to spot the differences. You may also simply want to gawk at Tsukasa and Nagato. This coming from 2ch, by way of Syu’s Quiz Blog.