Mac users bought the computer they did because they found the experience more appealing. Bringing an application across from Windows with minor tweaks simply won't resonate with this sort of user. ... Maybe the most important thing you will ever need to know about Mac development is this: Mac users will generally favor an app with a better experience over the one with more features.
The whole "write once, run anywhere" idea comes from and resonates with managers and engineers who are out of touch with their customers. Fundamentally, you need to decide who you are trying to help. If you are trying to help, say, Mac users be more productive and on their platform of choice, while still interoperate with the rest of the world, then that dictates certain realities. If you are expanding to the Mac platform simply to "increase your market coverage" then you might not have the right mindset needed to build something Mac users will like.
Not everyone gets this, and that's okay, the market has a way of helping folks that don't get it. I remember back when we were working on Office X, right as Apple was moving to Mac OS X. We spent some serious time and money to study and really tangibly understand who these "Mac users" were. The results were amazing and strongly pointed out how different the Mac customer was compared to the Windows customer. That has not changed. I don't think it's elitist or smug to say that Mac users value different things compared to Windows users. It's a fact. So, if you are going to try to sell software to both the Mac users and the Windows users, before you start, you better understand the differences.