So how do we judge the perfect annotation? Obviously, spelling, punctuation, and grammar are essential. But that's just the beginning.
The first and last question you should ask yourself: Did I explain the selected text? Will the reader more fully understand the text with my addition?
There are different ways this is done. Sometimes only a simple definition suffices. Sometimes an acronym needs spelling out, perhaps with a quick description or embedded url.
The best annotations, however, give context. They explore the implications or repercussions of a text. They not only explain what, for instance, President Obama is saying, but also what he's not saying. They unpack the carefully crafted rhetoric of leaders. When appropriate, they debunk, they challenge. Or they an*lyze how the rhetoric might be intended to say one thing to one audience and something altogether different to another. The best annotations are precise an*lysis that, once added, the text can't live without. We're writing on the wall of history, not slapping up graffiti.
Additionally, they're fully linked to reputable sources. Wikipedia is sufficient sometimes, but you might take a look at the footnotes on the wiki article to find primary, journalistic or academic sources on which the wiki authors relied. And the degree to which you predict your annotation to be perceived as “biased” is the degree to which reputable sources should populate your annotation.
And to add another degree of difficulty, the best annotations accomplish all this for readers who know less than you. After all, that's the nature of the relationship between annotator and reader: “Here, I've done the research. Let me explain to you what Obama is saying. Here's what it really means.”
The annotation should also be attractive, when that's possible. A picture is not always necessary (and they're often unnecessary), but many times they're essential. The < h2 > header script sometimes livens a tate. Long quotes should be block-quoted for readability and overall tate aesthetics.
Basically, what I'm saying is that avoiding lengthy block paragraphs with no variation is a plus. Humans are smart, but we're not that smart--we still like pretty things. Attractive annotations invite the reader in a way that a 6-paragraph mini treatise does not.