* Needs an admin interface to allow for resetting the $class_id outside the code itself.
I'm not a fan of plugins with single setting option pages. But if I do take it *more* public, I'll certainly look at ways of expanding the feature set, which would require a number of settings to play with.
* Probably doesn't need to check for the DIV in the excerpt part.
Actually it does -- at least, to my way of thinking. Most probably do not provide an excerpt and allow WordPress to take it from the content. And in the event part or all of a div is displayed with the excerpt, it could be confusing since it would not even be blocked off by the div (tags having been stripped from the excerpt).
In any case, it certainly doesn't hurt anything running on the excerpt.
* Might want to expand for other identified elements such as SPAN.
Checked for expanding that options page.
* Needs a spiffy plugin name like ClassBlocker or NoClassFeed. ;>