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DisplayPort port is like HDMI. Without the royalty fees. I encourage you to check out Wikipedia on it for all the small details over HDMI. It's target is a plug for Home system and PC (replacing DVI).
As for USB, from what I saw. It's nothing spectacular. See it as "I need a second screen desperately but I don't have a video card with 2 DVI's, and don't know how to upgrade of have the money to upgrade the video card". Image quality is not near DVI (but I only saw 1 setup, so I don't know if it's the monitor that was bad, or something else), and again comes down to... can it do gaming? Let alone render HD movie content super smoothly as the video card would normally do.