Nouns. Not all nouns are weighted the same, but each noun you introduce into your blurb is one more piece of information your reader has to remember and keep an eye on. The more nouns you have in your blurb, the heavier it gets, the harder it is to keep track of and the easier it becomes to lose a customer.
Science fiction and fantasy suffer the most from noun bloat. This usually comes from the fact that they contain names that are unique to that series and are often times hard to pronounce. The more familiar customers are with the nouns you use the less effort it takes to keep track of what's going on. Take a look at these examples:
District Attorney, Travis Jones, has been invited to the Governor's Ball in upstate New York. Little does he know, the Secret Service is bugging his apartment and his girlfriend works for the CIA.
Ether Empath, Wyveller Malchi, has been invited to the Praetor's Terrene in the Magelenic Core. Little does he know, the Royal Shadow is bugging his pod and his cortege works for the PLG.
Both of these say the exact same thing. The first example is pretty easy to follow. We are familiar with all the nouns used. The second example is much harder to follow or understand what is going on. Again, each noun carries a certain weight. If we continue these starter blurbs and add in plot, conflict and another character, one of them is going to be too heavy for the customer to carry and they will drop it by clicking the back button. The blurb is too heavy to buy.
It's not uncommon when an author asks for advice on their blurb for me to comment on the noun overload. It's a mistake I see when they try to just cram in as much as they can, because they don't know what to focus on. Another issue with nouns is that many authors introduce us to them, but then ignore them. In the forgotten land of Westcheerios.... Then the rest of the blurb has absolutely nothing to do with Westcheerioes or why it's a forgotten land.
In conclusion, limit your nouns. Make sure the ones you do use are relevant to the story you are trying to get a customer to buy. Remember, the job of the blurb is to get the customer to buy.
Taglines. Many rookie authors go for taglines or catch phrases when tackling their blurbs. The problem with them is that they require vast exposure and popular culture to work. May the Force be with you. I'll be back. Fifteen minutes can save you fifteen percent or more. In almost every situation, a tagline isn't going to work for a self published author because of the lack of massive exposure.
Another shortcoming of a tagline for a book is that authors often pull some witty line from their book that they are proud of and try to show it off. In reality it's irrelevant, distracting and you can tell the author tried to force the blurb around it.
My second novel. I had a nice tagline. It was fairly relevant, catchy and set the mood. Everything a tagline should do. But I cut it, it wormed it's way back, got cut again and then I found the perfect little spot for it. Here is the blurb again.
Branded as an outlaw for his daring rescue of the aerial warrior Angela, Kail finds a new home for his magic abilities in a gearworks mining town. There people are willing to help Kail and his group as they too hold no love for General Therion’s advancing airship armada.
As devastating losses mount, the Eternal Gateway reappears, and the fight for its control is rekindled. Kail and his allies know the Gateway cannot fall into Therion’s hands if they are to prevent a dark future foretold in prophecy. With little resources left, word of a possible key to victory reaches Kail and Angela, but it risks their best chance to seize the Gateway on the temperament of one volatile mage and a man immune to magic.
Through time the Gateway returns a burned and unforgettable face; Xavier Ross has the knowledge of what is to come and lashes out at Kail and Angela for choices they have yet to make. He who controls the Gateway controls time, even death, and Xavier knows at the end of time lies the beginning of vengeance.
Were you able to find it? It's the last part of the last sentence: "At the end of time lies the beginning of vengeance." In earlier versions I started the blurb with it. It was very hard to let go, but there ended up being a place for it. As for using them in your blurb, my recommendation is to avoid them like the plague. Very rarely does a dedicated tagline work. Even the one I used is no longer a tagline, but I was able to embed it into the blurb.
Taglines and catchphrases are totally awesome for Twitter. Even without the social requirements to make them work. They work there because the medium of Twitter demands short catchy dialogue.
Take a moment and browse the top 10/100 lists on Amazon. Take note of how many use taglines or catch phrases and their impact on pushing your mouse to the buy button, or if they are even relevant to the rest of the blurb. If you find a blurb that doesn't excite you, count the number of different nouns.