What are the most common copywriting sins?

Well, five pop into my mind instantly. All of which can be easily avoided:

Don’t be selfish:

Your audience have a choice. And if they sense that your content is all about you (‘me-oriented’ copy), not them, their choice will be to go somewhere else.

So don’t focus on the ‘I’, ‘we’, or ‘us’. Instead, construct sentences using ‘you’ and ‘your’. You-oriented copy attracts readers, keeps them interested, and cues them for action. It’s not about you, it’s about them, so focus on what your readers want, rather than simply what services you offer.

Don’t be self-absorbed:

You don’t buy from you, others buy from you. Your customers don’t really care about your business and your troubles nearly as much as you do— so keep you content focused on their business challenges. Keep it customer-centric.

Don’t be deceitful:

See selfish, above. If you don’t tell customers the truth, it’s probably because you’re selfish. How urgent can your needs be that you would sacrifice your future to get something now? Developing long-lasting and profitable relationships with customers is all about building trust.

Don’t be inconsistent:

Your customers are bombarded by information, daily; so they’re not paying that much attention. But when they do, it helps if you’re communicating in a similar way to what they have heard from you before—in terms of facts, messaging, tone of voice, language etc.

Don’t be lazy:

You should make your copy easy to read, and sometimes that means using the proper mechanics of English, such as when to end a sentence, when to use commas, dashes, colons and other punctuation. You should understand sentence structure, such as the need for a subject and a verb, how to use prepositions and conjunctions and phrases. Given that, don’t feel compelled to follow every rule of English composition. While you should not try to impress readers with your brilliance, you don’t want them to think you are illiterate.