<title> that accurately describes the subject of the page, contains the most important keyword (or two, or three if required).
<meta name="description"> that describes the page accurately in more detail, and motivates the searcher to click.
Note that I said "page," not "site." Each page should have a unique and accurate title and description.
One and only one URL for any page or resource: 301-redirect www to non-www or vice-versa. Strongly preferred: Link to "/" and not to "/index.php" or "/index.html". If you must use "/index.php" or "/index.html", then do it consistently, and never link to "/". Whichever URL you pick, 301 all other variations to that URL.
Consistent and correct server response codes, Expires, and Cache-control headers. No 404s unless the requested URL is nonsense; Intentionally-removed page URLs should return 410-Gone. Except for the canonicalization redirects mentioned above, no 301 or 302 redirects unless the URL has actually been changed; Don't use them as part of the "normal, expected function" of your site.
Search engine friendly URLs. Despite a lot of progress in indexing dynamic sites, static-looking still URLs do better, and are easier (if kept short) for people to remember.