Are there interesting graph classes where the treewidth is hard (easy) to compute?
Treewidth is NP-hard to compute on co-bipartite graphs, indeed the original NP-hardness proof of treewidth of Arnborg et al. shows this. Additionally, Bodlaender and Thilikos showed that it is NP-hard to compute the treewidth of graphs of maximum degree $9$. Finally, for any graph of treewidth at least $2$, subdividing an edge (i.e, replacing the edge by a degree $2$ vertex adjacent to the two edge endpoints) does not change the treeewidth of the graph. Hence it is NP-hard to compute treewidth of bipartite 2-degenerate graphs of arbitrarily large girth.

The problem is polynomial time solvable on chordal graphs, permutation graphs, and more generally on all classes of graphs with a polynomial number of potential maximal cliques, see this paper by Bouchitte and Todinca. Note that in the same paper it is shown that the set $\Pi(G)$ of potential maximal cliques of a graph $G$ can be computed from $G$ in time $O(|\Pi(G)|^2 \cdot n^{O(1)})$. Also, Bodlaender's algorithm determines whether $G$ has treewidth at most $k$ in time $2^{O(k^3)}n$. Hence, treewidth is polynomial time solvable for graphs of treewidth $O((\log n)^{1/3})$.

It is an outstanding open problem whether computing the treewidth of planar graphs is polynomial time solvable or NP complete. It is worth noting that the related graph parameter branchwidth (which is always within a factor 1.5 away from treewidth) is polynomial time computable on planar graphs.