XHTML is supposed to be semantic, e. g. it indicates clearly that "this is a quotation", "this is an abbreviation", etc. The only thing Dreamweaver can do is put cleanly indented <div></div> brackets around everything, to make sure no search engine leafing through the page will ever have the slightest clue about the content. Which is bad.
I'm not sure what you mean exactly. XHTML is a derivative of HTML4 that conforms to the XML standard. It doesn't add anything not already present within HTML 4.01. And to the best of my knowledge there is no provision within xhtml for custom tags that give context or semantic information, that's what XML is for..
And the DIV & SPAN tags are a normal part of writing html now, the idea being to get rid of the random spew of formatting tags with the far cleaner CSS. How it impacts the search engines I wouldn't know, I'm just going based on the best practices suggested by the W3C.