A friend who works at a record store told me this story.
A customer came in one Sunday afternoon. This was one of those "special" customers (i.e. the kind that make clerks hide in the back office until he leaves). He strutted in decked out in full hip-hop gangsta attire--baggy jeans, baggy shirt, white basketball sneakers, chains around his neck. He headed straight to the Hip-Hop section and started barking out questions to my friend--"HEY YO, YOU GOT ANY [insert random underground rapper name]?"
After about an hour of this, the guy asked to use the phone. My friend handed him the phone. The customer dialed a number and waited for a couple of seconds.
"YO DAWG!" he said finally. "YO, WHERE YOU AT? I'M AT THE RECORD STORE, DUDE! COME PICK ME UP!"
The customer hung up the phone and left. A few minutes passed, then one of my friend's coworkers came over to him.
"Hey, so that guy who was just in here?" the coworker said. "I think his mom came to pick him up."
Hopefully, this story made you laugh (I certainly did when I heard it). If you did, you probably laughed at the way this guy was talking to his mother on the phone. Granted, some mothers are more gangsta than others--my mom loves Scarface--but most of them probably won't answer to "Dawg."
That's the point I want to make with this story: Whether you're calling your mom or writing for the web, you should use an appropriate tone.
People often change the way the speak depending on whom they're speaking to. You're probably more polite and restrained talking to your 90-year-old grandma, for instance than you are when you're out having a drink with your buddies/girlfriends.
When you're writing web content, you should do something similar. Take some time to picture who you want to read your words--their age, their education level, their cultural background, etc. Once you have that picture, try to tailor your writing to fit that audience.
For example, say you're writing content for a veterinarian. Her clients are typically families and their pets. In this situation, you'd probably do better to adopt a warmer and more personal tone.
On the other hand, say the vet's clients are 20-something businesspeople with no kids. In this case, you should probably write in a brisker, more professional tone.
While you should think about the people you're writing to, you should also keep in mind who you are (or, if you're writing for someone else, who your client is).
Nothing turns off readers more than a dishonest writer. If you write in a way that doesn't feel true to you or your client, you risk looking like a con artist or that "special" customer. Try to find a balance between your personal style and that of your intended audience.
In short, don't be no sucker. Keep it real, yo.