In order to simplify parsing (and in order to enable parsing a well formed document without knowing its DTD), XML threw out much of the syntactic sugar that SGML had to offer such as implicitly closed tags. I.e. Many people think that <tr><td>foo<td>bar... is invalid HTML because of missing </td>. In fact, it is perfectly valid because the HTML (SGML-)DTD specifies that a <td>-tag implicitly closes a preceding <td>. Most think it's a browser-hack. It is not.
What both XML (and SGML) add over S-EXPR is support for expressing grammers, a well-defined validation mechanism and quite good support for coping with different character encoding.