Casual doesn't mean bad, it just means less 'depth'/simpler controls than the mainstream. Given a mainstream platformer is something like Assassin's Creed which is in 3d, has more weapons and more detailed controls than Trine (not to mention a fully acted and cutscened story) I don't see how it can be anything else.
That's a rather bizarre view of video-games though, isn't it? I don't gauge complexity by production values or control scheme. An indie game like VVVVV is widely regarded as a hardcore experience, even though it has an exceedingly simple control scheme, has no voice acting, and stylized retro graphics. Machinarium is also regarded as a deep adventure title, not at all casual or shallow. Point-and-click adventure games typically have simple control schemes because, well, they're point-and-click. And there's several of those already on this site. Beneath a Steel Sky is, for example, a precursor to Machinarium, and both play in similar ways with similar settings, so I am not seeing how Machinarium is antithetical to the GOG ethos. I don't think simple controls or an "indie" tag renders a game casual. Like that other poster said, casual suggests that anyone, including non-gamers, can just pick up the game and have fun. VVVVV or Super Meat Boy would have most non-gamers up the wall. I think tabulating complexity or depth in a video-game by counting the number of buttons used or items offered, which is basically what you're describing, is kind of odd.
I agree with what you and SLP said. I think labeling Machinarium and Trine as casual is weird. I don't see it at all