It seems that, in an attempt to compete with TripAdvisor, HolidayCheck needs to offer generous incentives to get you to review.
You will earn 150 Etihad Guest miles or 100 Miles & More miles for every hotel review you post on HolidayCheck. You can find full details of the Etihad version on the Etihad website here.
As Etihad allows you to cash out your miles for ‘virtual’ Visa cards via PointsPay at 0.4p per mile, you could effectively earn 60p per review by doing this even if you had no interest in the Etihad miles! PointsPay does charges 99 cents per withdrawal transaction though and this is unlikely to be worth the trouble.
Other airlines also let you earn miles via HolidayCheck. These are:
airberlin topbonus (200 miles per review)
airBaltic / pins (150 points per review)
There is a cap of 10 reviews per month and you are allowed to backdate reviews for stays up to 24 months ago.
Note that American Airlines appears to have been dropped as a partner at some point over the last few months.