I'm interested in the social, cultural, and political landscape around me, and I think I've "got a book in me" on the topic. A major source of material would be the writings of everyday people in places like Tweets and letters to the editor.
Q: Is making a "value judgment" about a public statement someone have any legal risks?
Q: Would I need permission to quote a publicly made available statement, such as a Tweet?
Think it right, everything has legal risks, since anything can originate arguments that may seem as valid for somebody. That's the reason why court houses exist: to judge if an argumentation is legally valid. I know this might be a quite extreme affirmation but, yet, it's a valid one.
In the first place, you have the legal right to have and express opinion. If you read or watch something, you can make a value judgment on it. There's a difference, by the way, in giving your opinion - what's a product of personal believes - and affirmation - what's a product of a fact.
About permissions, that's why licenses and terms service are for. Check Tweeter Terms of Service!
You retain your rights to any Content you submit, post or display on or through the Services. By submitting, posting or displaying Content on or through the Services, you grant us a worldwide, non-exclusive, royalty-free license (with the right to sublicense) to use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute such Content in any and all media or distribution methods (now known or later developed).
Also
This license is you authorizing us to make your Tweets available to the rest of the world and to let others do the same.
You can use tweets as much as you like as far you respect the terms of service. If you want to use other sources of data, you will need to check the license of that source.
Of course, you will always have the right to express your opinion on a book even if it forbids reproduction, as far as you don't use any of its content.
Q1: Make sure you made it clear you are stating opinions, not facts. "He's a thief" is a libel. "In my opinion, he's a thief" is free speech. Also, until sentenced, the culprit is merely "allegedly culprit". IANAL so I'm not sure if more rules or exceptions to that don't apply.
Q2: Make sure to read up on Fair Use. It's not much of reading but it's important. Use of quotes in news reporting, criticism and commentary (and other categories not quite applicable here) is allowed. You are still required to assign credit where it's due, and note, they can still sue you for copyright violation - it's then your duty to prove before the judge that Fair Use exceptions apply. It's a protection against losing the lawsuit, not against the lawsuit.
I am not a lawyer, but my understanding:
In the U.S., you generally have the right to express your opinion on any subject you like. If you talk about specific people, you have to beware of libel and slander laws. Basically, you can express opinions about people, but if you make statements of fact, you must have some substantiation that those facts are accurate. For example, if you say, "I hate Senator Jones", that's a statement of opinion and should be absolutely protected. If you say, "Senator Jones accepted a bribe from XYZ Corporation", that's a claim of fact and he could sue you for libel if it's not true. If you falsely say that someone committed a crime, that's always libel. Other statements about them could be libel if they can show that it causes them harm. Like if you say, "Mr Smith was born in Maryland" when he was born in Connecticut, the statement is provably false, but unless Smith can show that this error causes him some harm, he's not going to win a law suit.
That said, there are growing numbers of "hate speech" laws that you have to be wary of. Like, a representative of the US Justice Department recently told a newspaper that criticizing Islam on Facebook and other social media is a violation of Federal civil rights laws. http://www.tullahomanews.com/?p=15360
Regarding quoting public statements: As SF says, you need to read up on "fair use". Basically, you can use short quotes of statements that others have written or said for purposes of commentary, criticism, or education without getting their permission. The law is vague because the concept is vague. Like, there is no legal definition of "short". When someone sues, a judge decides it on a case-by-case basis.
