I would say a majority of why he was unpopular relates to the Korean War and General MacArthur. There is probably a lot more to it. In Merle Miller's book Plain Speaking, Truman goes on at length about the impact firing MacArthur had on his legacy. MacArthur was a pretty popular general in the second world war and was selected to command UN troops in Korea. After a series of events Truman and MacArthur were butting heads, so Truman had him removed. It became a very public issue. There were Senate hearings etc and it became an issue where common people chose sides. In addition to his troubles with MacArthur there were also corruption issues with the IRS. People taking bribes and that sort of thing. By the time he left office his approval rating was just as low as George W Bush.
I think he's looked back on more fondly is for 1 - ending WWII, establishing American dominance with nuclear weapons and 2 - The Marshall Plan. Plain Speaking is a pretty interesting read, you get a feel for what kind of guy Truman was and there are definitely parts I like about him and parts I dont. I can relate to the dichotomy