For me the most important considerations are;
Can I use an email address to send and receive funds?
Will the processor allow me to link my bank savings account for deposits and withdrawals?
Do merchants and affiliate programs accept this processor?
Do they operate in the USA, Canada, the EU?
Can I setup a donate button/payment button on my website?
What restrictions do they place use? (Paypal won't let webmasters be involved in certain niches)