Use this phrase to compare your expectations of something before you experience it with your opinion afterward. For example:

It wasn't as easy as I thought it would be.

This means that before you did it, you thought it would be easy. Then you learned that it wasn't so easy. Here's another example:

The flight wasn't as bad as I thought it would be, but it was still pretty uncomfortable.

You can talk about other people's opinions as well:

It wasn't as big as she thought it would be.

And you can replace "thought" with other verbs:

They weren't as fresh as I'd hoped they would be.

The opposite of this phrase is "___ was ___er than I thought it would be":

It was a lot harder than I thought it would be.