As long as you don't have any modified WASD key bound, hitting a modifier key (Shift, Alt, Ctrl) shouldn't stop movement... I can continue running forward via W while using my Shift 1-4 skills, etc.

That said, in most MMOs I've played, you can hold D, then Shift, and it will continue the command as simply D until released and re-hit, so that even if you have a Shift-D bound as something else you can always continue to strafe if desired, whereas this game will always recheck all keys immediately upon using a modifier, making that impossible.