It is worth pointing out a few things:
* The average individual does not understand the law.
* The average individual thinks they have a solid understanding of the law.
The question was posed: Is a phrase really trademark-able?
The answer, like most things involving the law (and or lawyers) is "Sometimes."
If you look at that link, most of the comments are fanboyish, and many of them confuse copyright with trademark. However, there are at least two notably useful comments. Specifically, the comment from "Elizabeth Potts Weinstein" is on point, but it is worth quoting "Jeff G"'s response (which he quotes from: http://cyber.law.harvard.edu/metaschool/fisher/domain/tm.htm):
“The standard is “likelihood of confusion.” To be more specific, the use of a trademark in connection with the sale of a good constitutes infringement if it is likely to cause consumer confusion as to the source of those goods or as to the sponsorship or approval of such goods. In deciding whether consumers are likely to be confused, the courts will typically look to a number of factors, including: (1) the strength of the mark; (2) the proximity of the goods; (3) the similarity of the marks; (4) evidence of actual confusion; (5) the similarity of marketing channels used; (6) the degree of caution exercised by the typical purchaser; (7) the defendant’s intent.”
Now, for simplicity, you can usually sum it up the trademark usage check process like this:
* Does the use of the trademark fall within the set guidelines for Fair Use? (in the US, this is defined by the terms of the Lanham Act, and the interpretations established in case law)
* Is the trademark being used in a way that might confuse another product in the same space? (e.g. I can't call my new soda "Croca Cola" or "Popsi")
For how Leo used it in Twittering, they easily meet both criteria (it is clearly Fair Use, and the scope of the Twitter is well outside that of published works, no one will confuse a 180 character Tweet for someone else's book.)
So, even if "Fedora Is Awesome" is trademarkable as a book title, as long as no one tries to use it in their own book title (e.g. "Why Fedora Is Awesome") without permission, or make confusing derivations as book titles ("Fedora Is Awwsum"), they're almost certainly fine.
Recently, I had to notify someone that their chosen name for a rebuild of Fedora, "Cowboy Hat Linux", was not okay. If we run it past the same simplified tests, it is clear why:
* They're not explicitly using "Red Hat Linux", so they aren't trying to qualify as a Fair Use case. (They were, however, using Fedora's trademarks to describe itself.)
* The term is clearly in the same space as Red Hat Linux (it is a Linux distribution), and it is confusing (as would be terms that rhymed (Dead Rat Linux) or terms with single words replaced (Fish Hat Linux).
The individual was amenable to the situation, and ended up dropping the "Hat" from the name to avoid confusion (Cowboy Linux is acceptable). In addition, I did not hit him with a form letter to Cease and Desist, we had a conversation about Fedora's concerns, and came to an happy resolution.
So, to bring this back to Leo's Tweet, is the phrase "Feel the Fear and Do It Anyway" trademarkable? Yes, within the scope of its use as a book title. Does Leo's Tweet infringe upon that trademark? Nope.
IANAL, I just help Fedora out with their legal issues, and serve as a liason to Red Hat Legal on such matters