This makes it harder to "theme the skin", adding an #id to the selector means that no selector can override the style unless it uses at least the same number of #id components in the selector, creating very lengthy selectors. It is more themer-friendly to use classes.

!important should only be used when absolutely necessary, for the same reasons as above and also for accessibility reasons (people with bad eyes can use !important to force a minimum font size or high contrast, which is then overriden by the sweaver skin where it also uses !important).
If you delete all the !important markers in sweaver-grey.css the editor looks exactly the same, so it's definitely not necessary here.

More info: