WhatsApp has been one of those sad apps where the UI was essentially ported from iOS, and hasn't taken advantage of the Android design language, but that is all about to change. The beta has gone with a full Holo-theme redesign, and we have to say that it looks quite good.
WhatsApp is currently free for one year, and 99 cents per year after that. If you've already paid, you won't have to pay again for using the beta version. As of right now, the new redesign is not in the Play Store, so if you want to try it out, you'll have to sideload it from the WhatsApp website.
source: Android Police