Every time I go to the beach, I see runners on the boardwalk pounding away merrily, chugging along at a leisurely pace which, in part, is influenced by the speed restrictions on the boardwalk. Every time I see them, I wonder, how much does the pounding on the boardwalk hurt in the long run? The slow speed definitely makes it easier on the legs but overall you're still pounding concrete and I think that has to hurt over a period of time. When I was in my 20s, I could run anywhere and the next day would still be the same. Now that I'm in my 30s, where I run and how long I run can and does affect how I feel the next day. Indeed, if I run on concrete and/or tar roads, I am more likely to have sore legs the next day than if I run on dirt trails. Especially, if I run hard.
The other problem with concrete running is the monotonicity of the surface. Over time, this can lead to weaknesses in muscles. Variety, as that found in trail running or running on other uneven surfaces, gives the muscles, ligaments, tendons, and other assorted unpronounceable parts, a chance to work out and in the process, strengthen themselves. Running on the boardwalk fails to provide this variety, unless of course, the boardwalk is covered with potholes or little sand dunes from overnight tides.
Nonetheless, this is all still anecdotal evidence. I wasn't able to find any studies on the effects of running on concrete for several years. A proper research study would definitely make for a stronger case. If you do know something like that, please drop me an email.