DrTeeth: It looks like the last two updated games in my account have not actually had the games themselves updated, but only the bonus content. Even then, it is not very easy at all to tell what has been updated.
Gog.com should give more info on what exactly HAS been updated.
Generally speaking at least, Judas will usually post a sticky at the top of the particular updated game's support forum, under the Community link, especially if it's an updated installer or available patch. That's often the first place I'll take a look when I get the "updated games" notification, if I can't see specifically what's been updated. Often he will also post what is included in the update, or if it's too long a list, he'll supply a link.
I agree though, it would be nicer to have an even better system in place, to let us know just exactly what's been updated at a quick glance.