Waiting for the perfect case to walk through your door is a losing proposition. If you only limit yourself to taking the cases with which you feel entirely comfortable, you will be stuck working on a few simple types of cases for your entire career—never developing the legal experience necessary for your career to progress. This will make you bored and unmotivated. I have had far too many referrals from people who are totally capable to handle the cases they send me, and they just missed an opportunity to build experience and bring in revenue.
To avoid being sentenced to a static career, I advocate using the 10% Rule (which is something I just made up for this post). For each case you take, you should be unfamiliar with how to do about 10% of it. This is enough to keep you learning and keep you interested as the case proceeds, but not so much that you will find yourself in violation of Rule 1.1 of the Rules of Professional Conduct. You still need to be competent to take the case, but the lack of a bit of the necessary knowledge or experience is not a problem and should not deter you from taking the case.
One of the best ways to learn is through experience, so one of the best ways to build your legal knowledge and experience is by taking on tasks you have not performed or mastered.