There are many content sites on the Internet. As a freelance writer, I've written for many of them. Some of them have editors who review material, and some don't. I can say from experience, however, that the editorial team at Mahalo does the most thorough job that I have experienced.
Mahalo editors check for formatting, ensuring consistency in layout and style which standardizes the content's look and feel. They also check for grammar - are commas in the right places? Are words used correctly? Is everything spelled correctly? Does the writer know the difference between affect and effect?
We also check for AP Style - since it's not intuitive, a QC of a page may send an editor to the AP Style Guide as part of their review.
All of those things are pretty standard, however. Mahalo editors also check to see if the material flows in a logical manner that is understandable to the reader. Is language simple and concise? A reader's time is precious, so the page needs to be scannable, with only relevant information - no filler or repetitive language to waste time.
The most important part of the quality control check is for accuracy. Citations are checked - both to ensure the source is authoritative, and that the information on the page accurately reflects what the source says.