But maybe I’m just weird, I also prefer Lightroom’s interface to Aperture’s ;)

In general, “the apps on Y look and work just like the apps on X” isn’t an attractive argument to anyone but an accounting department. People who have a choice are presumably using the Mac (or Windows, or Linux) because they feel it is better, more productive, more usable than the other platforms. The native human interface and platform capabilities are what do that.

A Windows user who switches to the Mac is going to have to learn how Mac applications look and feel and their standard user interface conventions anyway. Having some applications on their system that look and work the same as those on Windows — and differently from the rest on the Mac — can actually be disruptive to that learning experience.

On consistency: Screw consistency! Actually Apple isn’t too consistent with their apps. iPhoto uses light blues for ‘selected’ states on buttons, other apps have different conventions. Look at the difference between GarageBand, iPhoto, Aperture, Final Cut Pro!

The issue at hand is not so much that Picasa is inconsistent. It’s downright ugly.

An application’s interface has the purpose of making the application’s features accessible to its users. For a Mac version of Picasa, those users are assumed to be Mac users. You read that right: the main target for a Mac application is not Windows users (or converts… whom I guess are like tourists?)

Read Apple’s Human Interface Guidelines if you want an exhaustive list of reasons why an application should not sport an interface that looks or behaves significantly different from its native OS. For those who haven’t read it (I’m looking at you google), it should suffice to say the main reason against non-native controls is that they increase the amount of time it takes to learn how to use or even discover an application’s features.

A special addendum for OS “converts” is that an application that uses a non-native interface may mislead said “converts” into believing that their new OS behaves like their previous OS. This could stifle a user’s willingness to learn the conventions of their new OS and lead to frustration when lack of that knowledge unnecessarily impedes their work flow.