Start another Borderlands playthrough

Edit

Well since people are actually reading this, there's a little bit more information on each point.

The answers in order:

• Yes - All quests reset - The game begins just as though you had started at lvl 1 (minus the character selection, so you'll have to do everything over again (including the tutorial!)
• No - You keep everything - This is pretty straightforward, what you have on you at the end of playthrough 1 stays with you. However, items in playthrough 2 are scaled (see below on scaling). So you'll find the same named weapons but they will have better stats, so don't just ignore them.
• Yes - Mobs scale to your level -It is expected that after completing your first playthrough you will be around lvl 35 (give or take). Therefore when you start a new game, the mobs will have 35 added to their level (again give or take a level). Which is why specific enemies (like Crawmerax for example) have a set level. If you're looking to do the Secret Armory of General Knoxx, you should be lvl 50ish (about 2.5 playthroughs).
• No - You can go find it in playthrough 2, or you can access playthrough one with your current character (so if you have gained a level in playthrough 2, that level sticks when going back to playthrough 1). This includes any subsequent playthroughs.

Hope that helps!