Something I've noticed recently with the search function: there is no way to find games which use periods in between the letters of the main title (S.T.A.L.K.E.R., M.A.X., F.E.A.R., etc.) unless one actually knows other details of the game -- a secondary part of the title (e.g., Call of Pripyat) or addendum to the title (such as "Platinum"), the developer/publisher, some genre descriptors, etc. -- and uses them when searching. Omitting the periods does not help, replacing them with spaces does not help. I had a hell of a time finding F.E.A.R. Platinum and S.T.A.L.K.E.R.: Shadow of Chernobyl via the search.
This is quite irritating for users who already know one of these games is here, as we have to use circuitous methods to actually FIND the game page unless it happens to be on sale, in which case there are generally links to the page in various places. But this does no one any good during the vast majority of the time when the game ISN'T on sale, and thus doesn't have convenient links from the front page or a promo page. It also means you'll be even less likely with these titles than with others to sell copies at full price. This problem is arguably worse for GOG -- and, obviously, the publishers/developers -- as people who are new to using GOG (or who only visit the site occasionally) may search for a game, fail to find it (because of an unintuitive search function that doesn't know how to deal with punctuation marks), then shrug and go buy it somewhere else. You have probably been screwing yourself out of sales for a quite a while because of this; maybe not a massive number, but they're still lost sales (many of them lost FULL PRICE sales).
I may have mentioned this some time ago, but it would also be good if '&' and 'and' were interchangeable in the search engine (once you figure out how to make it not ignore punctuation, that is :) ). Currently, if I search for "with fire and sword", it gives me no results; if I search for "with fire & sword" (or "with fire sword", which is apparently how the search treats it anyway), I'll find what I'm looking for. This also applies to the Mount & Blade series in general (that pesky '&').
I also have noticed what I would call a bug when I use the "Go back" function of my browser (Firefox 35.0.1) to return to a previous search result: it sometimes starts to duplicate the search parameters bar all the way down the page, leading to a near-freeze Firefox. I haven't been able to duplicate it when trying, and of course it's easily avoided by just doing the search again, as opposed to trying to revisit results of a previously-run search. Not necessarily support-ticket-worthy, though I'll file one if it'll help. :)
Don't let this wall of probably negative-sounding text convince you that I don't think you're doing a good job, however. As far as signing up new publishers and getting decent games, you've been doing a hell of a good job the last couple months (and presumably the few months before that, when a lot of the groundwork for some of this recent awesomeness was no doubt laid). But I do think the store's search functionality is rather important, and it would probably work best to put some time into it BEFORE Galaxy launches, rather than after, when you might (one hopes!) have a large-ish influx of new users to deal with.
[sigh] This ^ is why I don't post very often in this thread. Every "feedback" message turns into a verbose wall of text. =|