When I have traveled, I have used TripAdvisor and Lonely Planet as well as using referrals when arranging accommodations. But it's also important to talk to everyone who does a lot of traveling and definitely read the reviews before booking. Although with europe email list reviews, I have found there are many varied opinions on the same hotels so it is preferable to go with the opinion of the majority. As in everything, some people are just naturally more critical.
When I travel, I don't expect five star accommodations if I'm only prepared to pay for three or four star. For myself, three or four star is quite sufficient. However, my main requirements are safety, cleanliness and convenience, first and foremost. Other things factor in as well but they are secondary.
When we were booking accommodations for traveling in Europe, we were going the hostel route and so our secondary requirements were slightly different. But whether staying in hostels or hotels, top of the list of my secondary requirement is whether there is wireless internet connections available. In Europe and Korea, it seemed to be everywhere. In Australia and New Zealand this wasn't always the case.
Other things to consider (which may or may not be important to you) are: is a breakfast included; how central is it to the attractions you most want to see; is there parking provided and if so, do hotel guests have to pay for it; if you happen to be on the fourth floor or higher, is there an elevator; is there a restaurant in the hotel; is there a pool; is the hotel in a quiet area and, is there a staff member available twenty-four hours a day.
When staying in hostels, another concern was whether bedding and towels were included; if the rooms were mixed gender or single; if three people, for example, were traveling together, were there three beds in a room and, were the rooms ensuite or were bathroom facilities in the corridor. When staying in a hostel, we thought that the inclusion of bedding and breakfast were important requirements.
So before booking, it is important to ask all these questions to ensure that you get what you want. Nothing would spoil a trip faster than arriving and finding that the things you feel are most important are not available. You can get answers to these questions by either asking someone you know who may be familiar with the hotel, checking online, asking a travel agent or telephoning or emailing the hotel directly.