Watch out for that NDA. Be wary if a company asks you to sign a non-disclosure agreement, particularly if it's vaguely worded. Ask for clarification first, be sure you're aware of all the specifics, and get a copy of it so you have a reminder. An innocent blog post about your job has the potential to get you into a lot of trouble.
Companies often have preconceived ideas as to what their audience wants. It's a tricky business trying to figure out what sells and what doesn't, but that often results in being closed-minded and unwilling to experiment with new material. More ideas are rejected than accepted.
Never assume a personal opinion is private. Whether you write it on a blog, web site, or Facebook, people and companies have ways of finding you out even if you don't post under your real name. Your opinion can have major repercussions for your job (even though it shouldn't). To be safe, don't post it.
Expect others to take credit for your ideas. From plagiarizing a blog post to outright stealing your original idea, there are those who are so lacking in imagination that they won't hesitate to grab your valuable work and pass it off as their own. Aside from asking the person to remove the content, there's little you can do to protect your work.
Your co-workers might not be trustworthy. This ties into the previous two points. It's fine to brainstorm and discuss ideas but be careful what you say or share. There's almost always one person in the group who's trying to further themselves at the expense of others, and will tattle to the boss when anything controversial comes into the conversation.
All this isn't to say that you can't trust anyone, but be careful who you share your information with.