I see both points of view, but when it comes to plugins that affect styling -- and yes, that would include forms (though forms could be switched out if a use wants more features) and getting the theme to work right "out of the box" so to speak, I think that building the options into the theme really makes sense.

As others have stated, particularly Bill, Steven and John -- the more things that the user has to install to get stuff to work -- or the more things that have to be updated, the more hassle.

Case in point. I tons and tons of staging sites on my MAMP server running on my Mac. This way I can have a million different installs of various things and keep trying stuff out, breaking stuff, etc. However, I also like to have a mirror of my current sites, for development purposes. When it comes to creating a local mirror, I have to install WordPress, upload my themes and plugins that I use on my main sites over, activate them, make sure the settings are set correctly, etc., just to get to the point that I can have a good mirror. Now, granted, I can (and often do) just import in the MySQL file and mirror the databases, but even so, if I'm setting up a new testing environment, it's a lot of work.

And that's just for one theme. You add in other stuff into the mix, and it just grows from there.

Plus, what happens when the person who made your really awesome contact form disappears off the planet. Or your plugin for a certain graphical element isn't updated and breaks in future versions. My point is -- for stuff that you see visually, I think it does make sense to build it into the theme when possible. Sure, make it an option to turn it off or to override, but build it in so that the out of the box experience doesn't suffer.

When it comes to stuf like SEO, Google indexing, and other backend components, I'm a little less sure. I think building in a option for a feedburner URL or adsense code makes sense, those plugins are typically just injections into the header files anyway -- but for more in-depth stuff, it might not make sense to make it all about the theme.

Really though, I think that the future of WordPress is shaping up to be less about everyone change themes like they are socks and more about what Drupal is doing, which is to package different distributions around a certain concept. So you have your Thesis WordPress distribution that is your whole site in a box. And you can edit the CSS and other stuff and you can add things to it but the theme becomes a fundamental part of the backend, not merely a component. That's how real content management systems work and if WP moved in that direction, it would be a good thing.