The first is that it requires you to "register" a credit card. That means you need to give the carrier your credit card details to store permanently, rather than simply entering it each time you want to perform a top up. It's fine to offer that as an option for those customers who want it, but I don't see why you would make it mandatory. Personally I would prefer that they don't keep my credit card number on file for all time for security reasons and anyway I may well use a different credit card the next time I want to add value to this account.
The second problem is with the design of the screen (shown below) for entering the credit card details.
I entered my credit card number and then selected "American Express" as the card type. At that point the system changed the format for the credit card number from the 16 digits used by Visa and Mastercard to the 15 digits used by Amex and in the process deleted the data I had already entered. Most people will do what I did - start entering the data from top to bottom - so the page should have been designed so that the selection of the card type was above the box for entering the card number.
Shame on you Vodafone Australia.
However to their credit, I should mention that my wife had a very good experience with them yesterday when she wanted to activate the voice mail on her account. She called their customer service number and was connected directly to an operator (can you remember the last time that happened?) who was very helpful (or that?) and the whole process, including configuring her greeting and so on, was successfully completed in less than five minutes (Ok, I know, you should still be navigating the recorded menu). I suspect hell would freeze over before Verizon matched this level of service.