Steam News offers a lengthy essay from Valve about their recent decision to put some obstacles in place to slow down the process of completing trades for Steam items to help prevent scams. Trades will still be completed instantly when protected by two-factor authorization using a mobile authenticator, but other trades will involve some extra time to make sure everything's legit. Here's the conclusion where they explain their reasoning:

Once again, we're fully aware that this is a tradeoff with the potential for a large impact on trading. Any time we put security steps in between user actions and their desired results, we're making it more difficult to use our products. Unfortunately, this is one of those times where we feel like we're forced to insert a step or shut it all down. Asking users to enter a password to log into their account isn't something we spend much time thinking about today, but it's much the same principle - a security cost we pay to ensure the system is able to function. We've done our best to make the cost as small as possible, for as few people as possible, while still retaining its effectiveness.

Hopefully this post has given you some insight into the problem, and why we've taken this approach. As always, we'll continue to read the community's discussions throughout the Steam forums and the web at large, and we look forward to hearing your thoughts.