Required for raiding

If you don't have the above addons, you may be replaced in the raid for someone that has. Some bosses are complicated and not having the correct addons will lead to wipes, less progress and higher repair bills for you and your fellow guildies.

Optional addons

OmniCC
OmniCC is an addon that adds text to items/spells/abilities that are on cooldown to indicate when they'll be ready for use.

OmniCC shine
Adds a shine effect when a cooldown finishes. Requires OmniCC to work.

Quest helper
QuestHelper tells you how to finish your quests in the easiest, fastest manner. It includes a database of quests, monsters, and items, telling you the exact location of that dang Perfect Satyr Horn you can't find. It also computes the fastest path to complete all your quests - and gain experience - as quickly as possible.

Atlasloot Enhanced
AtlasLoot Enhanced is an UI mod allowing for loot tables of bosses to be browsed whenever needed within the game. It can access full item tooltips, show drop rates, allow items to be chat linked and to use the dressing room.

Digits
Digits is a numeric head-up display of health and power (mana/rage/energy/runic). When you enter combat it will appear, and when you leave combat it will hide.

Mikscrollingbattle text
MSBT is designed to be an extremely lightweight, efficient, and highly configurable mod that makes it easier to see combat information by scrolling the information on the screen in separate, dynamically creatable scroll areas. It is a replacement for Blizzard's Floating Combat Text and Damage output.

Sunn viewport art
Sunn's Viewport Art allows you to change the size of the rendered world area (viewport) and adds textured artwork in the non-rendered areas. After there you can search for more sunn art pack by just google sunn art or dl it from the same page as this.