You can only have 5,000 people in your circles on Google+. You can also only add about 1,000 in a certain amount of time before getting an error message. This isn't brand new information, but it doesn't get talked about all that often, at least that I've seen.
To many people, this might not seem like a big deal, but consider two things:
A. Google just opened up Google+ to the public (without the need for invitation)
B. Google just started letting you share Circles.
With regards to A, a lot more people are going to be joining Google+. That means a lot more people to potentially add to your Circles. Access to many different viewpoints and pieces of information that you may otherwise miss. Why would Google, whose stated mission is to "organize the world‘s information and make it universally accessible and useful" want to limit that?
With regards to B, Circle sharing makes it very easy to quickly hit your limits. If you find a lot of interesting Circles others have shared, the number of people in your Circles can quickly increase. If you have a lot of interests and wish to follow Circles reflecting those interests, you may not be able to follow everyone you want.
You can certainly hit your 1,000 very quickly.
If Google wants your profile to be your identity, it needs to not stifle your access to content of interest, which can stand in the way of you being informed as you want to be (not to mention +1ing more content to send signals to Google search).
Of course Google has plenty of other products that give you access to content (search, Reader, Gmail, etc.), but why limit it in Circles? Especially if you're going to let people share them with each other.
"Hey, check out this awesome Circle of Googlers I created. Oh, you don't have room for a couple thousand more people? Nevermind. Maybe you can find them on Twitter. It's not as if they're posting more content on their own social network or anything."
What happens when Google finally launches brand profiles? If users want to add brands to their Circles, they'll run closer to their limit even quicker.
Google is said to be in talks to acquire Katango, which looks at your Facebook friends and puts them into Facebook lists for you. Presumably, Google wants to use the tech (or talent) to improve Google Circles.