Overall improvements

Drupal is generally complimented on its code cleanliness, so let's do it also right for the one-stop-shop theme that zen is. Code cleanliness will ensure a higher adoption rate and as a result more suggestions for improvement in an absolute sense . Below are my detailed suggestions :

Delete helvetica and verdana throughout: arial and sans-serif are in reality sufficient
Change colors to short-format and uppercase: from #ffffff to #FFF
Replace 0px by 0: delete superfluous unit for something which is zero.
Remove background: #FFF; from body in zen.css: a background is very rarely anything other than white
Change occurrence of table tr.odd to table tr.odd {
background-color: #DDD;
} (line 32 in zen.css)
Delete cog.png and modify administration panel h3 in icons.css accordingly: not a great image and does not add much
Convert all icons to png and change icons.css accordingly
Apply shorthand for comment-unpublished in zen.css: background: #FFF url(images/unpublished.png) no-repeat center 20px;
To create a faux column: add in zen.css to #page: background: url(images/bg-faux.png) 0 0; (need to create the image yourself because width depends on the width of the column(s) in use)

The remainder is how to create a footer-stick.
- additions to layout.css:
html {
height: 100%; /* footer-stick */
}
#container {
margin: 0 auto;
/*padding: 0 20px;*/
max-width: 1270px;
/* the remainder is footer-stick */
min-height: 100%;
position: relative;
height: auto !important;
height: 100%;
}
* html #container { height: 100%; } /* footer-stick */
#footer {
margin: 4em 0 -3em;
/* the remainder is footer-stick */
position: absolute;
bottom: 0 !important;
bottom: -1px;
width: 100%;
}
- additions to zen.css:
apply padding: 1em 0; /* footer stick */ to #footer