Anyone who was hoping for big changes to the Google Keyboard's functionality is going to be disappointed with L. The performance and features are almost identical, but it looks a lot different. Don't freak out yet – the old themes are still there.
The default look for Android L's keyboard is "Material." It uses borderless keys and a slate gray/pale green theme. I actually quite like it, but I've heard from a lot of you who feel very differently. Luckily, the advanced keyboard settings still includes a theme option. In addition to Material, you get white and blue themes.
The white theme is the KitKat keyboard – it's exactly the same as it was before, and it still fits into the OS fine. At least right now it fits in... maybe as Material Design picks up steam, the old theme will start to feel more out of place. As for blue, that's the Jelly Beam theme with our old friend #33b5e5 Holo blue. It's weird Google would continue offering this, having moved away from Holo so completely (it's available in KitKat too). The only strange thing UI-wise is the clipping around the floating text bubble when you're using swipe input. It seems too small for the lettering. Otherwise, it's a nice tweak to the UI. Nothing groundbreaking.