A feature on other sites that I miss here is when you post a link, the preview of the link shows up in the stream as the primary content. Any commentary you add, if long enough, gets folded under the Show More option.
D* instead hides the link preview under the fold and requires a user to expand the Show More to see both the link and the preview of the page.
I feel the other systems have the better system. The link is the main crux of the post. The commentary potentially is better or more impressive, but the link is the context in which the commentary is being made and should be at the forefront of a reader’s mind.
Many people actually want the exact opposite of your suggestion. For most people, the preview is completely irrelevant and they want it as unobtrusive as possible (I read hundreds of comments stating so). Personal commentaries make the posts worth reading, otherwise it would be little more than a news feed. Also, links are often not the main content of a posts but only extra references or sources. Not everybody mainly posts news articles. You’re the first person I ever read who wants the preview as the primary content.
Myself I prefer any commentary to precede link preview or reshare (there is no way to add comment to reshares yet but it is coming and I can’t wait for it).
The logic is simple - when you introduce something you usually say something about it beforehand. You don’t shove random objects to people, wait for their reaction and then explain. Well, sometimes you do but these scenarios are rather limited. Quite often the content you refer to can be questionable by itself and you might reshare something you don’t agree with and making right introduction is crucial for making right impression. Sometimes it is plain uninteresting unless put in context first - so without proper introduction it will just be skipped.
Right now if you want to make a “surprise” introduction you can just post the link and add your message in comments. We have more choice for content presentation this way.