Personally I don't think Joan has much personality. She's been shown about as much as Monica, I don't think either of their characters are fleshed out people. The difference to me is that Morse likes and wants to be with Monica (as much as he is able). Whether he doesn't want to "go there" with Joan because he is genuinely uninterested in her or because she's the bosses daughter is debatable, but as it stands he wants to be with Monica and I like what I've been shown of it.