Zones

Current hot zones
As of December 14, 2011:

Keys, flags and zone level requirements
Many keyed zones from the Ruins of Kunark, Scars of Velious, Shadows of Luclin, Planes of Power, Gates of Discord, Omens of War, Depths of Darkhollow, and Prophecy of Ro have had their key/flag requirements modified. All players that have obtained the key/flag will still be allowed entry into the zones, as will players that have met minimum level requirements for the zones. The zones and minimum level requirements are as follows:

Required Level 46
 * The Howling Stones
 * Old Sebilis
 * Skyshrine to Cobalt Scar

Required Level 55
 * Plane of Storms
 * Plane of Torment
 * Plane of Valor
 * Crypt of Decay
 * Sleeper's Tomb
 * Veeshan's Peak
 * Vex Thal
 * Locked areas of the Temple of Ssraeshza

Required level 57
 * Bastion of Thunder
 * Halls of Honor
 * The Temple of Marr
 * Tower of Solusek Ro
 * Drunder, The Fortress of Zek

Required Level 60
 * Plane of Water
 * Plane of Air
 * Plane of Fire
 * Plane of Earth
 * Ikkinz
 * Uqua
 * Yxtta
 * Kodtaz

Required Level 62
 * Plane of Time
 * Inktuta
 * Qvic

Required Level 65
 * Txevu
 * Tacvi

Required Level 68
 * Anquish
 * Theater of Blood

Required Level 69
 * Riftseekers' Sanctum

Required Level 70
 * Dreadspire
 * Demiplane of Blood

Required Level 72
 * Deathknell

Howling Stones
From the October 8, 2009 patch message:
 * Apparently while performing some end-of-the-summer cleaning, the doors within Howling Stones were left unlocked. It may be some time before a maintenance crew can be sent there again to lock the doors up tight. Use this knowledge as you will.
 * In addition, the spells protecting the entrance to Howling Stones have weakened, and adventurers of lesser skill and strength may find its halls now accessible.

Planes of Power

 * The Plane of Innovation requires that you be level 46 or higher.
 * The Planes of Power flagging guide - Note: This is now mostly obsolete, as being level 62 or higher will get you into almost all of the Planes of Power zones.

The Serpent's Spine
Removed the flag requirements for The Serpent's Spine (TSS) expansion raids to enter the raid zones as long as you are level 74 or higher. The person requesting the raids will still be required to have the proper flags to request the raid. - Patch December 23, 2008.

Secrets of Faydwer
Zones with key or flag requirements will now unlock themselves automatically when later expansions open (on standard and progression servers). Meldrath's Mansion and Crystallos zones are now unlocked on standard servers.

Seeds of Destruction
Korafax, Lair of the Riders (Kuua) and Discord Tower are locked until you complete The Void "E".

Sanctus Seru

 * AllaKhazam's Sanctus Seru page

Access to Lord Seru
When you port in with Arx key, turn left and go up first staircase. When you come to the first lobby at the top of the stairs, bear left (south) and head all the way down the corridor. You come to another lobby. There is a room directly ahead. Open the door and go in. On the left wall is a bookcase. Immediately after the bookcase the wall is fake. Pass through it to a similar room. Exit the room by its door and dead ahead is the clicky-portal to get to the start of the maze.

Maze: