What qualities do online merchants prioritize when choosing a payment processor?

Having sold/introduced 100s of merchant to payment processors, here is what we have found usually that holds true (in descending order of importance):

• Types of Cards Accepted and/or Other Payment Avenues accepted
• Costs per Transaction
• Cost for Returns
• Settlement of Funds (# of days)
• Holding Reserve (if any)
• One-time Costs
• Monthly Recurring Costs
• Territories in which the system will work
• Multi-Currency Options
• Native Integration with vast majority of Shopping Carts
• API Availability
• Customer Support
• Approximate time it takes to approve an application, and time to go live.
• Knowledgebase, FAQ and Online Forums section