Recommended gear levels for this instance is full T1/T2 gear with 220 hit/focus or 150 toughness.
Greenscale's Blight is the first raid instance introduced to the game and was available when retail launched. The instance is life based which means life resist will help, but it is not required. This instance is considered a tier 1 instance, but really River of Souls is similar when it comes to difficulty level. Recommended progression will be listed at the bottom of this page.
This is their entry level 20 man raid so the first few bosses are complete push overs which can be killed with less than 20 people. However, if you roll in with any spec you want then you may run into some trouble. Make sure you do a little bit of research so you have competent raid specs which will not hold you back from progressing. Consider Greenscale's Blight the "raiding 101" chapter before they begin to make more difficult content. If you are in the type of guild where they allow you to play whatever you want then simply ignore the part about competent raid specs and play what you enjoy.
Greenscale's Blight item level is a little lower than River of Souls and even Gilded Prophecy beats out some of the drops from the instance, but Lord Greenscale drops relic quality items which are very, very good. Clearing the whole instance grants you a reward of 15 Marks of Ascension which you can use to buy tier 1 raid gear. Also, the Epic Quest Saga item drops off Lord Greenscale which gives you a very powerful epic weapon of your choice. Make sure you bring at least one raid member who needs the quest item so it does not rot.
If you complete all acheivements towards Conqueror: Greenscale's Blight you will be rewarded with a Gweenscale in the mail. Gweenscale is a vanity pet you can summon/dismiss whenever you would like and it looks exactly like a miniature Lord Greenscale.
Recommended Progression (in blue are bosses located in GSB):
Duke Letareus --> Infiltrator Johlen --> Oracle Aleria --> Dark Foci --> Warmaster Galenir --> Prince Hylas (for guilds with strong healing)/Plutonus the Immortal (for guilds with strong DPS) --> Herald Gaurath --> Alsbeth the Discordant --> Lord Greenscale