I admire you for giving back to the community!
IMHO, if what I'm reading around this forum recently is any indication, most people looking for themes are novice users, and just want something that will work with minimal fuss. That would mean not a whole lot of deviation from normally-structured pages. To me a theme is more for the look, and less for fancy functionality, which a user can add with plugins and such.
Though better SEO would be great, I imagine your typical user will just install All-In-One-SEO anyway.
As long as your theme is widget-ready, it will be easy for someone to customize sidebars, for instance. You could just include some of the more obvious choices for those who don't use the widgets, such as Recent Comments, Categories, Archives, and Links.
I have finally began making sites with font replacement techniques myself, and for me, if I've covered True Type and also the silly EOD files that IE uses, I consider myself good to go. There would be backlash, I suppose, if no IE support.