The name to a large extent (IMHO). PayPal just sounds too casual and I'm sure many consumers instinctively don't want to transact through a service who's name implies friendship and informality rather than serious business.
Granted, it may be far less of a problem today as eBay and therefore to some extent PayPal have become household names but it certainly used to be the case.
Now, i'm not saying "Buy now using your Google Wallet" is the best formal business presentation you could ask for but Google have to some extent achived through other methods what less heard of payment processors achieve through naming - such as "SecurePay" etc. It's an emotional thing at the end of the day, we're an irrational bunch.
/on a side note, i'd have loved to have been a fly on the wall in the meeting where some not-so-clued up external branding consultant proposed "GoogleCash" as the name of their new payment service... :)