Graph problems which are NP-Complete on directed graphs but polynomial on undirected graphs
The first problem comes to my mind is the 2-path problem (or more generally k-path problem). Given $(s_1,t_1)$ and $(s_2,t_2)$, find two disjoint paths from $s_1$ to $t_1$ and $s_2$ to $t_2$, respectively. The problem is NPC for directed graphs but polynomial-time solvable for undirected graphs (although not trivial).