1.       Cancellation Rates:  Many hotel sites lure you in with low hotel rates until you review that the rate is non-refundable.  They will probably list their refundable rates which may be what you could have booked directly with the hotel directly.
2.       Rate Variation:  It pays to shop several web sites for hotel rates as they night per stay can vary up to $200 more per night, depending on the quality of the room and facility.  Trivago.com is a good site to check first as it is a directory site for popular travel sites such as expedia.com, booking.com and hotels.com.
3.       Breakfast:  Check to see if the nightly rate includes a complimentary breakfast.  With breakfast typically averaging $20 a person, this can add up, especially if you have a party of two or more in your room.
4.       Guests Per Room Fees:  In the United States, it used to be that you paid for the room and that was it.  It didn’t matter if you were a party of one or a family of four.  This is quickly changing and most hotel chains are following Europe and other countries where rates depended upon the number of people in the room.  If you more than two for your room, may be best to see which hotel doesn’t upcharge significantly for each additional guest.
5.       Room Size:  When booking a room on-line, especially in Europe, don’t assume all rooms sizes are equal in size.  You may have booked the deal of lifetime in London, only to discover upon arrival that your room is the size of a large closet.
6.       Resort Fees:  Hotels have jumped on this sizable fee which is not readily visible with hotel sites.  You need to check the fine print to see if the particular hotel you are booking will add a resort fee per night.
7.       VAT Tax:  In Europe, it’s very common to see this “value added tax” to your hotel rate.  This can increase your rate over 20% in most cases.  If you are on a budget and felt that the “low rates” shown for a four-star hotel in Europe were a steal, think twice as once you add the VAT, resort fees, and any other fees, you may be exceeding your budget requirements.
8.       Reviews:  Most hotel sites include reviews, comments and ratings from past guests.  It’s worthwhile, however, to get an independent perspective such as from TripAdvisor.com and see the overall rating assigned to that particular property or reviews for comparison purposes.
9.       Reward Points:  If you book with different hotel sites, you may not be eligible to earn reward points, which can add up over a period of time.  Some travel sites have set-up their own rewards program for clients such as expedia.com and hotels.com.
10.   Booking Changes:  If you need to make a change to a reservation, review their cancellation or other policies with changes.  When you book directly with a chain, you may have more flexibility such as only a 24 or 48 hour notice to cancel or make other arrangements.