I think the medals tend to be nicer for anniversaries, idk. Of course medal design is a matter of preference no matter what.
I seem to remember back in the old days they used to make a bigger deal of some anniversaries, like the "20-mile surprise" (which was a disappointment for many people after the hype prior to the race) in the 20th anniversary of the marathon. IIRC in the 25th anniversary there was a little special photo opportunity at mile 25. Not really relevant for the 15th anniversary of a half marathon though, and even those special things seem to have dwindled away anyway.