The Wave experience can quickly become frustrating as your conversations get longer. It can take a couple of mouse-clicks to reply in the middle of a blip and then again to finish the blip. If you’re constantly shifting from keyboard to mouse and back again, your productivity can quickly suffer.

But take the time to learn some of these handy shortcuts and you’ll find the user interface less baffling and the usefulness of Wave will increase (I guarantee it!). For example, you can use “Shift + Enter” to:

Reply to a message at the end of a wave. The new message will appear at the same indentation level, at the very end of the wave.

[… But also has the …]

Same function as ‘Done’ button - signifies you are finished editing your addition to a wave.
Just this one shortcut has made my experience of Wave a thousand times better. You might find something that helps you out too!

Keyboard shortcuts [Google Wave Help]