First look at spelling, grammar and content:

When you first look through the site, are you struck by any grammar or spelling mistakes? If you notice a few, there is no harm in pointing them out.

If there are quite a lot, it is up to your discretion whether to point it out in your website review or send the person a private message about it. Sometimes it is obvious that the website owner is not a native English speaker. Other times, it may be carelessness. Either way, you can always suggest proof reading it again (reading it out loud is an easier way to catch your own mistakes) or have someone else proof read it. Of course there is always spell check which may catch many mistakes, but not all.

What do you think about the content? Is there a flow? Is it thorough? Do the author's points come across easily? If so tell his/her that. If not tell his/her that as well. Any author will want to know if his/her writing is clear or not. :)