Nothing we've done over the past 4 years should give you reason to worry and there's plenty of reason to believe that Elsevier doesn't want to piss off our community.
Choice quote: "For that reason then I co-founded PeerJ, an Open Access journal, with one aim of never being in the position to take shit ever again from a closed publisher."
I'm not 100% behind the Mendeley hate right now, but they're against all odds. While everything I've heard from the team so far, including on the prominent HN threads, has been reassuring and positive, it's still just words.
People are making predictions about the future right now, and no words will be able to sway public opinion. Only action, and time. It seems Mendeley (or, as of now, Elsevier) are betting on the latter, because there has been no action, only PR.
It means that a week ago CEO could make the same promises and be heard seriously, but the same CEO promises right now are not worth the paper they're not written on.
Now at any point in the future all the Mendeley 'openness' promises can be easily broken by Elsevier, so they either know that all the promises they're making now are empty (i.e., lying intentionally) or they don't know that (then they're naive and incompetent as general managers).
Promises can be made by those in control. Leaders in public companies can be considered in control. Leaders in privately held companies can be considered in control if their public vision and roadmap is aligned with the owner. But if the owner has a clear interest to change the roadmap - then it would be imprudent to rely on such a roadmap. Elsevier has a reputation of intentionally sabotaging projects (say, gov't initiatives) that should bring openness - if they fool you twice, shame on you.
And for broke, internet saavy students, free stuff with a promise of a new groundbreaking trend in science was enough to lure them.