This is a little less about reviewing and more code standards but I'm a firm believer in functions only doing one thing. I've caught so many bugs by enforcing that during my code reviews. Determining "only one thing" is kind of by experience where you should draw that line but I find if a function is pushing 25 lines of code or more or has more than one indent it's suspicious.

It's more work to refactor and keep things clean enforcing this, but boy has it made my life better. It also makes the code ultimately easier to review as you have to keep less in your head at once for context too.