Dimensional Gateway
Magical gateways between worlds similar to the Dark Portal. However, where the Dark Portal was securely fixed as a doorway between two temporal worlds, Azeroth and Draenor, with a single terminus on both planets, Dimensional Gateways are less stable, and are tunnels between the worlds, as opposed to mere doorways. This feature is part of what caused Draenor's explosion. The Dark Portal was only stable because both Gul'dan and Medivh worked to pierce the veil between the worlds. Ner'zhul had no such partner in opening his Gateways, hence their inherent instability.
There were four dimensional gateways, left over from Ner'zhul's experiements, outside of Magtheridon's Black Temple. They brought demons in from the Twisting Nether which Magtheridon brought under his control. Illidan Stormrage shut down the Gateways, which cut off the flow of demons and ended the extra stresses on the remnants of the planet.
It is likely that Kel'Thuzad opened such a gateway, or a variation thereof, in Dalaran to summon Archimonde. The gateway used to summon Hakkar the Houndmaster and the other demons in the War of the Ancients was closer to the Dark Portal than a Dimensional Gateway- the terminus of that portal was located on Legion-controlled world.
The Gates in The Burning Crusade
Several of the dimensional gateways that Illidan closed can be found in Outland in the Burning Crusade expansion. One can be found on the shores of Marshlight Lake in the west of Zangarmarsh, the second can be found at Twilight Ridge in the west of Nagrand and the third can be found at Death's Door west to Thunderlord Stronghold in Blade's Edge Mountains. The gates are not functional and seem to have been abandoned for a long time.
References
The portals to other worlds make up a collection of portals of which Chris Metzen revealed in the Developer Panels that Illidan closed these in Outland. Nevertheless, MagusRogue, a World of Warcraft RPG Game Designer from White-Wolf revealed the following: "The portals were shut down by Illidan, actually. However, some of the quests in the Burning Crusade Expansion have you re-opening these and going to new dimensions, fighting the legion in new worlds. Should be awesome." Source: MagusRogue, a World of Warcraft RPG Game Designer
"One of the things we're most excited about," says Metzen, "is the chance to create environments unlike anything you've seen in previous games. We really want to ramp it up for the players now. We're saying, 'Hey, you've graduate, you're over level 60 now, so let's get crazy with some really alien worlds and creatures." Even better, this is something that's infinitely expandable for us in updates and expansions down the line. There can always be more portal worlds." Source: Computer Gaming World, December, 2005 by Jeff Green
The portal at Twilight Ridge and Death's Door are likely one of these portals to other worlds.