Google SEO. Everything else is arbitrary and up to the admin themselves. Though Trash Can Forum and Fit to Page are pretty necessary plugins too. Really sucks they don't have either by default.
Trash Can Forum can be done by default simply by creating a hidden forum and a custom moderator tool. Very simple. Fit to Page is perplexing. It's called a dynamic width. Why would you need a plugin for that? So basically the two "must have" plugins you want are default MyBB if you know what you're doing.
Well, then I learned something (though I only listed Google SEO as a "must have"). Besides, as a not-as-experienced person, the plugin would work fine for me.