I'll reiterate that this is not the fault of the WhatsApp application. The ART verifier was blocking the installation of this app, but we are going to fix ART to address this issue. I'll update this bug when the fix is in AOSP.
The issue with installation failures for Dalvik or ART causing an app not allowing the app to be installed in the future is a serious bug, but not a new issue in Android 4.4 KitKat. The only solution that I know of for a non-rooted device is to factory reset (aka wipe data). If you root, removing the /data/app directory seems sufficient to allow reinstall.
I've heard other people say the same - that a switch from ART back to Dalvik messed up a few apps and the only solution that worked was a factory data reset.
If you are rooted you should be able to delete the folder for whatsapp in /data/data/whateverTheWhatsappFolderIs but you probably need to be rooted for that. I just tried to look in /data with Astro and it comes up empty, so I do think you need root for that solution.
Somebody mentioned that they could successfully install from the play store on the web, but I'm not sure if I buy that.
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