I have just been listening to the video talk by Steve Reichgut of Google, here:
www.joomla.org/component/option,com_jd-wp/Itemid,105/p,426/
One issue he raises is compatibility between the new component he has just developed (for 1.0.x, then on to 1.5 legacy & so on) and 1.5 templates. He is a component virgin, so his talk is very interesting, but one thing he mentions is that with 1.5 you can load so much functionality in to your templates that in the future compatability issues are likely to occur between add-ons and templates. See for example RocketTheme templates, that (even on 1.0.x) are loaded with Javascript functionality, complex positioning, menus and even their own core-replacement modules (required for the template to work).
So whereas we have all been concerned about clashing add-ons in 1.0.x, now we also have to worry about templates! This problem is likely to escalate, as template developers suddenly grasp the potential of 1.5 templates and go bonkers with it.
I have just picked up a client using a RocketTheme in 1.0.13. It's a very clever piece of work but quite horrible to customise, because it has quite a stranglehold on how it wants to look and work.
What do you think?