Thessaloniki was a pleasant surprise for me. I hadn't thought much about it before, but it's really a very clean and modern metropolis with a nice view of the sea (especially from the top of the White Tower) and a much milder (and less smoggy) climate than Athens. Its WH monuments are scattered throughout the city, but are easy to find and represent a good cross-section of the city's history from ancient Greek and Roman to Byzantine and Turkish times. The Byzantine churches especially are very interesting, but also the remains of the Roman Palace and the Triumphal Arch merit a closer look. I wouldn't go to the city just for its WH value, but there are many other reasons to go there, and seeing its historical buildings is definitely a good way to become acquainted with Thessaloniki.