You could send and receive email with a gmail user without being a gmail user - in fact it made people jealous to do so and was great advertising.
You couldn't participate in a wave unless you were on wave. (The spec was open, but obviously in the early days there were no other implementers. Also the non-open UI made a big difference).
I really thought they were going to fold wave into gmail, so that everyone on gmail would automatically be able to participate in waves and access them through the same interface as they did their emails. That would have been amazing. It was also just outgrowing its initial crashiness when they killed it.
