The initial worries I've seen expressed are that it would open the floodgates to being spammed by all and sundry. It's worth noting that you have have to opt in to the new service.
If you're concerned and want to check, then just go into your settings and select "security and privacy". At the bottom of the page there will be an option like this:
If you don't want to receive DMs from people you don't follow, just make sure the box is unchecked.
A few people have questioned why anyone would want to get DMs from complete strangers. If you run a busy customer service feed then I can see that it would be advantageous to be able to get DMs from people without having to ask them to follow you. Other than that, the uptake seems to be limited.
What do you think? Will you be opting in?