One of the traps of the system, the up hill battle. If you had more slots, you'd play more, but if you played more you'd have more slots. Gold to gems conversion, buying character slots when they're on sale, I've got 2-3 of every class without ever paying $ for it, unless you count that time they gave a slot for people around the F2P conversion, I forget the details of that, so it might count as me having paid money.
They limited the initial allotment to 5, basically one per race, which is odd. It's not insurmountable by any means though.