Active instance limit was reached, this is so frustrating.... No, this issue now has been updated according to Blizzard blue post.

The limit on active instances was originally implemented as an interim solution to ensure that all active instances perform smoothly. Within the last few weeks, we have configured and optimized our infrastructure to better handle the instance load, and many players are no longer experiencing issues with instance creation. However, to incorporate these upgrades across every realm is not a minor undertaking; it requires fairly complex coordination with third party affiliates who actually host the hardware we utilize.

This reconfiguration is already complete on more than 40% of the North American realms,the optimized realms are listed below, and the situation has noticeably improved on these realms. We are continuing to work towards the implementation of this reconfiguration for all realms, which we expect will resolve the instance creation issue for all players. We expect the optimization for all realms to be fully completed within the coming weeks, and would like to assure everyone that every effort is being made to ensure that this is completed as soon as possible.

Aegwynn
Akama
Aman'Thul
Arathor
Area 52
Auchindoun
Azjol-Nerub
Azuremyst
Barthilas
Blackrock
Blade's Edge
Blood Furnace
Bloodscalp
Bonechewer
Boulderfist
Bronzebeard
Caelestrasz
Chromaggus
Coilfang
Crushridge
Daggerspine
Darkspear
Dath'Remar
Dawnbringer
Draenor
Dragonblight
Dragonmaw
Draka
Drak'thul
Dreadmaul
Dunemaul
Eitrigg
Eldre'Thalas
Exodar
Feathermoon
Firetree
Fizzcrank
Frostmane
Frostmourne
Frostwolf
Galakrond
Garithos
Ghostlands
Grizzly Hills
Gundrak
Gurubashi
Hakkar
Jubei'Thos
Khaz Modan
Khaz'goroth
Kil'jaeden
Kilrogg
Korgath
Kul Tiras
Malorne
Mug'thol
Muradin
Nagrand
Nathrezim
Ner'zhul
Perenolde
Proudmoore
Rexxar
Runetotem
Saurfang
Scarlet Crusade
Sen'jin
Shadow Council
Shadowsong
Shattered Halls
Silver Hand
Silvermoon
Skywall
Smolderthorn
Spirestone
Stonemaul
Stormscale
Suramar
Terenas
Terokkar
Thaurissan
The Scryers
The Underbog
Thorium Brotherhood
Tichondrius
Uldum
Vek'nilash
Velen
Windrunner
Zangarmarsh