The current layout of Hyjal as it exists in the game is quite linear, which seems to indicate that it is intended to be a raid instance, and not a regular zone. Also supporting its intended design as a high-level raid instance is the fact that the only currently known entrance to Hyjal is a green raid portal located at the end of a long trek through Darkwhisper Gorge in Winterspring, which is currently the highest-level non-instanced area in the game. Within Hyjal, there is another raid portal, in a cave similar to Onyxia's, which some believe is going to be an encounter with the great dragon Deathwing.
The announcement of Hyjal's inclusion in the Caverns of Time has generated speculation that modern-day Hyjal may be redesigned as a non-raid instance. To many, the prospect of two Hyjal raid zones, one in the present and one in the past, seems highly unlikely. While this may be Blizzard's plan, some believe that the inclusion of Caverns-Hyjal may have superceded plans for modern Hyjal to be a raid instance, and that it will instead be refitted to serve some other purpose.
I All Mount Hyjal is an instance, making it the biggest outdoor instanced zone in World of Warcraft. The entrance is at the southwestern corner of the Darkwhisper Gorge -- within southern Winterspring region. The instance gate is closed by a wooden-door. You can see through the door, that the zone-in whirlpool is green, meaning 40-players raid. Here is where the epic battle of the forces led by Thrall , Jaina Proudmoore and Malfurion -- united by Medivh's spirit -- defeated Archimonde and the Burning Legion. Expect it to be an intense high-end content. The Darkwhisper Gorge is guarded by Elite Felguards, Succubis and Felhunters, so it is obvious Mount Hyjal has a strong Burning Legion presence.
There is no official info from the Developers of World of Warcraft concerning Mount Hyjal -- the present version. However, there will be a past version available through the Caverns of Time. Two Mount Hyjal instances? -- you wonder.
Hyjal, shown on the Wow Main Map but not accessible, is a rather large area that appears to be far along in development.
Hyjal is an as-yet open zone that is visible on the WOW main map. The area is surprisingly complete and contains many, many interesting features.