A few:

• Dilution. The founders and employees will own less. This may be worth it, but founders often forget just how expensive each funding round is.  More here.
• Loss of Control. Not a huge deal until after you raise a few rounds and provide a few board seats, but still a real issue. Each round you sell, you control less. Again, it may totally be worth it. Just something to understands.
• Some Loss of Optionality. Investors are going to expect you to “exit” for 3x-10x the last round price. You can sell for less, but it’s not always easy. And selling for less than you raise? No one makes any money in that case.  More here.
• Harder to Pay Yourself The Profits (If Any). If you are lucky enough to be minting cash, and paying yourself a ton, you often can’t do that anymore once you raise funding. A niche negative, but a real one, and for example one of many reasons Mailchimp never raised any capital.
• Have to Commit to Growing at a High Rate. VCs will expect you to grow at a pretty high clip. If that’s happening anyway, it’s no big deal. But if you don’t plan to grow quickly, it won’t be much fun if you raise a material amount of capital.
• You’ll Have a Boss, At Least to Some Extent. This can be good, it can push you harder and make you more accountable. Or you may sort of hate it :). More here.