What unique advantage made PayPal so successful in its early days?

From an outsiders perspective, here is what (in my opinion) was unique about them when they were starting out:

• Payment by email. The email was the common denominator for payments and also served as the unique identifier (this was especially true, because to find the unique identifier worldwide, email was it!) Obvious yes, but brilliant at the time to grasp it!
• Account Association: PayPal was not trying to sell you their own currency or coins, they simply juxtaposed of what you were already used to, your bank account & credit card.
• Not having to Share - Security: You did not have to share your bank account / credit card information with vendors/merchants whom you did not trust. Just share them with PayPal and that was it. No one at the time had thought of this. Everyone was busy implementing credit-card acceptance forms on their websites.
• Verification: The small-value deposit was just brilliant. It was never used before to verify a bank account (to the best of my knowledge). Then using the user's own credit card to make that deposit. Again, brilliant, the transaction essentially became a Zero-sum game for PayPal. Then using the same transaction on the credit card to verify the card itself, again - brilliant!
• Simplicity: Go through the registration and verification process once, and then pay each other seamlessly! No one at the time offered this.
• Grow with the Web: PayPal grew with the web. As the Internet got more and more mainstream, so did payments and PayPal was considered (and in some circumstances) still considered the de facto standard of paying when it comes to the Internet.

Like I cited, everyone was busy trying to get their online credit acceptance approved by the few payment service providers at the time, PayPal was busy enabling payments for all.