No, seriously, I can't stress that enough. It goes for any other class in the game, really. Anyone can play any class reasonably well - by reasonably well I mean you can finish a heroic dungeon without being a burden to the rest of the group. You can get in the top 10 dps spots in a LFR group. You can get through all of the solo content in the game without any trouble.
But the trouble starts when people who know their class reasonably well start coming into raid groups. I don't mean to be elitist, but just being "pretty good" at playing your class just doesn't cut it. You may get carried through the first few normal mode bosses without people noticing, but when you get to later bosses or heroics, people WILL notice you struggling at the bottom of the meters or not pushing out the required hps or being too squishy to survive mechanics.
Raids are for people who are, and want to be, more than just "pretty good". It's for people who actually invest a bit of time into researching their class, knowing the nuances of different stat weights and enchants, knowing some of the more intricate rotations and when to use CDs, knowing at least the basics of min/maxing.
Of course there are the dedicated few who go above and beyond that - those would be your hardcore mythic raiders who are at the top of their games. But that requires nearly a job-like dedication to playing and raiding. The average player doesn't need to invest anywhere near that kind of time to be a competent raider.
I've been playing a rogue for nearly 6 years now, so I've picked up on some things here and there. I certainly don't spend anywhere near the level of effort the mythic raiders do, but I do browse the theorycrafting forums now and again. I stay on top of class changes as they happen and re-adjust my spec and stat weights accordingly. I buy the best level of gems/enchants that I can afford and spend some time at the practice dummy whenever I can't decide between gear options. I do know my class. My dps is usually above other players in my +/- 5 ilvl range and I rank in the 80-90th percentile according to the logs.
So when our group brings in a pug rogue who is pulling half my numbers at an ilvl that isn't too far off from mine (less than 10 ilvl), I have to wonder what this guy is doing wrong.
He was wondering the same thing, as he kept whispering me, "what are you doing different? How are you doing so much dps?"
Assassination spec is by far the easiest of rogue specs as it literally requires 4 buttons (mutilate, rupture, dispatch, vendetta). He told me he was doing the rotation correctly, I asked him a couple questions about CDs, he insisted he was using them correctly. So all I could do was shrug and suggest that it was gear issues.
Well, I linked the logs from the fights to a rogue forum, and the response was quite enlightening. Turns out this guy was doing a whole slew of things wrong - subtle, little things here and there, but added up can result in a ~20k dps increase.
For instance he was using the +200 agi flask, not the +250 agi one. It doesn't seem like a big deal, but why not spend the extra 50g to get more of your primary stat? He wasn't using a prepot or combat pot, which once again, why not spring the extra few gold for the big dps boost? His rupture uptime was only 90% (despite him assuring me it was always up), he was using a wrong glyph, energy pooling, wrong enchants, you name it.
So all this tells me that this is a guy who wanted to play a rogue, found that leveling to 100 was pretty easy, went and read some tips on noxxic and decided he was good enough to raid in a heroic pug. I get that everyone has to start somewhere, but I personally wouldn't have the balls to step into a heroic pug without having first shown that I can beat 90% of the other players in normal mode first. The last thing anyone wants is to get laughed out of a pug and blacklisted...
When we removed him from group (this was after the third week of him getting carried) he bitched to me in whispers. This leads me to think that age may be a factor too - I'm guessing he's pretty young. Which is of course an issue in itself - you have no business trying to get in a group of 30-something adults as it is unless you're super mature for your age or just show up, pull your fucking weight and never talk in vent (we'll never know how old you are and that's fine!).
Last I heard he got discouraged and quit his rogue. I don't feel bad for him, honestly. Some people are better off being casuals, and that's ok. Just don't expect to be carried through content by other people.