You can get the "jump 300 feet" achievement in the "inner workings" of Testchamber 19. The set up you need is in the room that has... like... 12 turrets in it. You can make an infinite jump (not fall) loop with your inertia.
I loved the story of Portal. It was simple, and absolutely perfect for a two-three hour game. I felt that all of plot holes were pretty much wrapped up by the end, like what happened to everybody.
If you have any questions about who the heck the main character is, you should check out
www.aperturescience.com for documents hinting at certain plot details not present in the actual game (although they are mostly joking and nature... sounding like something GLaDOS would say). Hidden in Portal (the actual game), there is a name and password written on a wall that you can use to access the site itself.
When you open the site, type "LOGIN" to begin. Then you can type in the name and password.