what level can you enter the dark portal
The vast majority of my characters have gone through the Dark Portal at 56 or 57, and the breadcrumb quest that you get at the Dark Portal that takes you to Thrallmar / Honor Hold (http://www.wowhead.com/quest=10254) requires 55. I have regularly picked up this quest at 56 or 57 on my way to Thrallmar / Honor Hold.