Good product copywriting is concise, clear and mindful of the many different devices that people use to access the Internet.
Good product copywriting also goes out of its way to not irritate a user.
Few things will annoy a user more than complicated instructions or sprawling sentence structure.
If a user has to think too hard to "get" what you're trying to say, then this creates a usability issue, and the copy needs to be reworded.
In other words: keep it simple, keep it civil and keep it short.
Don't say in 140 characters what you can just as easily say in 50.
Writing in an active voice empowers the user and adds to the product's authority. Use a steady and consistent tone of voice. Confident but never cocky.
Also, don't talk down to a user or cast actions they've taken in a negative light. It's mean.
Try and avoid complicated words or phrases. Product copy should attempt to clarify an action, not make it more obscure.
There are times when adverbs are unavoidable. But, more often than not, you can remove them, or a sentence can be re-written to exclude them, without losing the message.
When it comes to the product, be selective when and where you introduce humor. A user may return to a screen several times when using your product or website. What once was clever copy becomes annoying when you read it for the twentieth time.
Using idioms (common sayings that aren't meant to be taken literally) in product copy can be disorienting to users. Idioms also have the same disadvantage that comedy has: what is clever the first time you read it becomes tiresome when you read it twenty times.
Industry-specific terminology can be intimidating to users who are not familiar with those terms. Whenever possible, try and find common alternatives. If no common alternative exists, be sure to define the term for the user early on.
Puns are not universally loved. Avoid introducing puns.
Saying hello is always polite, but once a user is well inside the experience, we can cut to the chase.
Try and be conservative with how you use exclamation points. They can be useful in adding emphasis to a sentence, but an excessive amount of exclamation points can be desensitizing to a user. When everything is exciting, nothing is exciting.
When addressing a user in a personalized experience, use the pronouns "your" or "yours." This possessive pronoun usage has a dual purpose: It brings a feeling of personalization into the experience, and it introduces a stronger sense of ownership.
The global exception to this rule is using the first person possessive pronoun "My" as a navigation element when you are trying to describe an element i.e. "My Account."
Never mix "my / mine" with "you / your" because you're gonna confuse people.
User experience copy should focus on the benefit to the user. Excessive use of first-person "I" or "We" diminishes this emphasis. Your product or website is seldom "doing" something for the user. The user is discovering things for themselves.