Those wood gussets look nice but they don't do shit for stabilizing the joints. Having all the vertical supports be butt joints is down right scary. You need to add "sandwich" boards at each joint and some diagonal supports. The ramp also needs support in the middle and they should have been built as boxes. By that I mean the slats should have been between the main boards and perpendicular to the ramp surface.
Just so you know I'm not talking out of my ass this is the ramp my friend and I build in his back yard. It's 6ft tall. http://home.earthlink.net/~zymurgyjw...board-ramp.jpg