Xenosaga and the older Xenogears is a great series and changing the control systems and gaming interfaces between parts, even altering the style slightly was a great way to keep the series fresh and generated a lot of new interest when playing each part.

I think some elements of the original 4 .hack parts did get minor updates to their interface and control mechanics but not that much between chapters and quickly playing GU Vol 2 it looked, felt and played identically to Vol 1 which might be a good or bad thing depending, as you say, on how good the story in the series maintains its momentum and continues to introduce new elements into the story.

I do intend to play through the whole .hack series at some point so I will finally find out once I do.