Code review checklist

Code should respect the policy described in the style page.
Commit message summaries have to be descriptive.
Commit messages should be accurate: if it has deviated from top-level description, don't r+ without getting that fixed; see e.g. flip-i-mean-rev commit as an example of accidental deviation.
Almost every change should contain a test case as described in the testing page.
Code optimization should contain a bench case as described in the bench section of the testing page.
Look for commits that could be squashed.

General Suggestions

Don't do partial reviews. If you're reviewing a PR, address it completely. This will reduce the pending time of PRs.
Whenever something can be improved or should be changed, be as detailed as possible in your comments. This will help contributors that are not familiar with the code to understand better what you're saying.
Add references whenever it's possible. For instance, when a benchmark is requested, link the benchmark section to your comment, unless you're sure the contributor knows that already.