What you need to raid:

Consumables! Some might see these as optional but in reality, not using them means you are not getting the most out of your character on a raid.

I continue to see players show up for Kara with little or no consumables such as mana or health pots or buff food/potions. If you're not sure what is best to bring, ask your KM or really any experienced raiding player. Pots, wizard oil and buff food give Sweetbelly an extra 120 spell damage.

Buff potions are broken down into Battle or Guardian elixirs. A Battle Elixir will increase offensive stats such as strength, attack power of spell damage. A Guardian Elixir will increase stats such as Intellect or Stamina. So a healer or caster could use both Elixir of Draenic wisdom to increase intellect and Elixir of Healing Power or Adept's Elixir to increase +healing or +spell damage/crit, respectively. They last 1 hour or until death.

Flasks count as both a Battle Elixir and Guardian Elixir but offer a much greater buff and duration of 2 hours, plus they persist after death. They are expensive compared to regular elixirs, but worth it.

Don't forget oils or stones for weapons.

There really is no excuse not to have pots and food for raids now. They are relatively cheap to produce (especially since we have mats for many of them in the guild bank) and of course gold is easier to come by through dailies. Some players will give pots out in raids to help out, but presuming upon this generosity is rather selfish.

Reagents: If you can do group buffs, please bring enough reagents. It slows things down considerably having to wait on a priest or mage to individually apply 30 minute buffs instead of the group buffs.

Enchants. I also continue to see people showing up for raids with few enchants on gear, or the wrong ones. Enchants can be problematic because of the cost of mats. Still, we have mats for some enchants in the guild bank, and many enchanters have extras, so you can probably work something out to at least get some critical enchants on gear. IF you're saving up to enchant an epic you're wanting from Kara, that's fine. But at least get a cheaper enchant on the gear you currently have, rather than none at all.

Socket gems. Except for meta gems, there's really no reason to have empty sockets in gear due to expense. We have plenty in the guild bank. Make sure you have the best ones for your class. In some cases, foregoing the socket bonus is better. IE: +Spell  or +spell/+hit gems are better for a shadowpriest even though a piece of gear might offer +stam or +crit socket bonus. Shadowpriests don't need either of these as much as raw +spell damage, and +hit if not already hit capped.

Bandages. Basically, if you don't have bandages you shouldn't bother asking for an invite. No, the healers can't always be there to heal you: sometimes you do have to stop dpsing to patch yourself up. Only exception of course is the tanks. But if you see the tank taking so much damage that the healers have to focus on him (or the healers are getting low on mana) don't get mad if you die because you didn't get a heal and didn't bring bandages. We lose a hunter on a boss fight, we lose some dps. We lose the MT, we probably wipe.

It's little things like these that give players, and the raid, a little extra edge. You may think "well, that pot only gives me XX amount more AP and is a bother to make". But overall, it adds up, especially when everyone in the raid is properly buffed. The result may not be immediately noticeable on a meter. But in the course of a raid it means downing mobs and bosses a little faster, healing people a little less often etc.