It looks as though Guardians actually has a decent chance of scoring the year's biggest opening based on last night's figures. I will admit that was never a possibility I seriously considered. But with sparse competition and an unusually slow summer season it's conceivable the movie could knock it out of the park, box office-wise.
As BoxOfficeMojo points out, it's far from a sure bet. It's certainly possible that the Thursday numbers were disproportionately good because the film's built-in fandom was unusually motivated to see it. But it's probably worth noting these numbers are a hell of a lot better than those for Pacific Rim a little over a year ago, a film which did end up having only a niche appeal (in America). So, at the very least, the movie's unlikely to be a bust as some feared (I personally felt it would be more of a mild success, similar to Thor or Cap's first outings).
So there you go. Guardians of the Galaxy, biggest opening of the year (maybe).
SATURDAY UPDATE: Based on last night's figures, it looks like Guardians (probably) won't score the biggest opening of the year. But it's absolutely set to make the August opening record (the closest competition is The Bourne Ultimatum, which opened to approximately $70 million) and it has a decent chance of beating The Winter Soldier. And there's still a small chance the film could surprise, if families come out in bigger numbers than expected (one indicator that might happen is that the film's ranking extremely well at Cinemascore).
Either way, Guardians is definitely a success and has proven Marvel can expand beyond the standard superhero tale into something more than a little different.