Automating the posting of content across various social networks (Facebook, Twitter, Google+, LinkedIn, etc.). It would take a very long time to do this manually. We would have to write our own software and figure out the APIs to do this any other way.
The reporting didn't work particularly well for our clients' needs. Most of our clients were paying too little to warrant in-depth reporting, so it wasn't really a big issue. There are a lot of great reporting modules that our clients just couldn't take advantage of due to their small size.
There is a limit (200) on how "tabs" one user can create. This hasn't yet been an issue but it may end up being one.
There's always room for improvement (and Hootsuite is always improving), but overall the product is very easy to use. And whenever we get stuck, they provide pre-recorded tutorials, help articles and/or hands-on training.
Google Docs would be great since we build our content calendars using a Google Docs template provided by Hootsuite. It would save us a lot of time exporting and importing batches of content for each client.