People have said time as #1 so I'll go with a supplemental #1
#1 - A capable computer/accessories. You need a computer to "flawlessly"handle what you have coming to you. This includes any graphics or potential issues any new raids may cause you. You can't drop out to 1FPS for 4+ seconds or have an internet connection which isn't top tier. You can't be jamming buttons to get a single response from your keyboard or mouse. All of this needs to be reliable. This has caused me to step down from raiding and caused me from losing a lot, more than anything else so I'll put that at #1.
1Mbps download .25upload. 9800Nividia card / 2.3Ghz core 2 processor/ raided at 15 FPS max low with only projected textures.
Still took huge amount of rank 1s and minimal mistakes if any. Hardware a crutch
#2 Class knowledge and performance capability. You need to know your class and spells inside and out. You need to be able to know what would or should be best suited for when and be able to use it. You also need to be able to and willing to FULLY utilize your entire toolkit. This means taunting a boss from a distance and bubbling, or giving a raid member a Salvation, or using a cleanse when none is available, or utilizing any utility you have to help your raid progress. If you have something to offer the raid OFFER IT. Yes, your DPS Is important and you're expected to maximize on that, but you should be able to trick and max your way through encounters and help ensure a kill by not just pulling your weight, but then some.
I'd rather make sure the slow guy dragging his ass out of the huge aoe survives if hes pretty low and i have no faith in a healer when I have an infinite mana pool. Feel like grips are highly underused priest wise.
#3 Going with #2 in many ways is a capable and functional UI. You have to know what t look for where to look for and when to look for it and be able to maximize on your classes abilities with minimal interference to your roll! The more you do beforehand means the less you have to do later! And that's a good thing. It needs to be clear and concise and functional. Someone once said to be someone should be able to understand your UI within 5 minutes of logging in. I don't know how much I share that sentiment but I think it's worth some weight.
75% of total wipes in progression are from retards having a bad UI and can't see shit because ADDON MALFUNCTION or their shit isn't setup in a useful manner. Debuffs should be clearly visible if not the player is a horrible failure at their character.
I know it's the top 3 things but there is a smaller 4th that I'd add.
#4 Attitude and raid etiquette. You can have fun and mess around, but you need to be able to sit down and shut up when things need to get going. You also need to have the attitude, mentality and patience to sit there until a boss is down or raid is called WITHOUT complaining (within reason). Nothing killed me more than trying to progress and pushing and then someone getting a piss poor attitude which just annoyed everyone and ended up with an endless circle jerk of nobody wanting to be there anymore. You have to be willing to get it done and not be satisfied until it is. If it's not you need to be willing to figure out what you can possibly do to help yourself, or others, make it happen.
To each their own. on a difficult boss people need to stfu imo esp if they fuck up alot or die in the early phases of shit that every already has down.
The attitude of trying to parse when it means not dpsing an add or being a dumbass while there is STILL PRGORESSION is one of my biggest player peeves. Just one shot the goddamn farm boss so we can work on shit that matters.
That said I wish I had a raid capable computer. WTB computer part donations