Imagine an article with many revisions and pending changes enabled: A, B, C, D, E, F, G...
A is an approved edit. B,C,D,E,F,G are all pending edits.
B is horrible vandalism that the subsequent edits did not fix.
You are a reviewer, you go to review page by clicking a pending review link. On the review page you can accept— thus putting the horrible vandalism on the site. Or you can "reject" which throws out the all the good edits of C,D,E,F,G by reverting it to A.
To quote someone from IRC: "this seems like its going to make vandals even more effective because all they have to do is make one edit in a string of ten good ones, and then the entire set has to be thrown out"
But that isn't true at all. You're not confined to the review page, you simply go to the edit history, click undo on B, and then approve your own edit (it won't be auto-approved because G wasn't approved). Tada.
This completely non-obvious to people, because the only options on the review page are accept or reject, and it's already causing confusion. This is a direct result of the late in the process addition of the review button, — trying to fit the round-peg of a revision reviewing system (which we can't have because of the fundamental incompatibility with single linear editing history) in to presentation-flagging system square hole that we actually have.
I don't know how to fix this. We could remove the reject button to make it more clear that you use the normal editing functions (with their full power) to reject. But I must admit that the easy rollback button is handy there. Alternatively we could put a small chunk of the edit history on the review page, showing the individual edits which comprise the span-diff (bonus points for color-coding if someone wants to make a real programming project out of it) along with the undo links and such.
In the meantime I expect enwp will edit the message text to direct people to the history page for more sophisticated editing activities.
(Thanks to Risker for pointing out how surprising the pending review page was for this activity)