The main part of the WHS is the riverside area Pier Head and Albert Docks, the Pierhead area (pictured) has the most famous building ensemble in the Royal Liver, Cunard and Port Authority buildings. The Albert docks are along the Merseyside a little further and are based around the large enclosed docks, which were one of the first in the world. In this area there are several free museums, the Merseyside Maritime Museum, is the best one to head to for an introduction to what makes Liverpool a "Maritime- Mercantile City" it also has some interesting exhibits on migration, especially of the Ireland- America route, and the lower floor is dedicated to the slave trade which the city played a large role in.
The site also encompasses parts of the rest of the city, which has some reasonably grand buildings, and the odd monument here and there, but on the whole I was left wondering if this was really a site worthy of being on the World Heritage List. It is important but I don't know if it is of universal outstanding value.
The city is nice, if a little scruffy, and is worth visiting if you want to see that England is not all just Royal castles, it has a distinctive culture and accent and it is a good place to experience, especially if you are interested in the Beatles, but I don't think it is exceptional enough to be a WHS.
I managed to spend an afternoon in Liverpool after heading up from Ironbridge Gorge, which is about an hour and a half drive away. If you don't have a car it has plenty of transport connections, it is about 3 hours by train from London or 5 hours by Bus.