Most group content in WOW is tightly regulated: five-mans are tuned for ... errr ... five people; raids are designed for ten or twenty-five. If you've got 11 people available for raiding tonight, someone has to miss out. You can't just wander up to an instance and help out, either - if your name's not on the list, you're not coming in.
That's a deliberate decision on Blizzard's part: raiding is intended to be hard, so that the most skilful players get the best rewards. The down side of this approach is that it's a very non-linear reward system. If your guild is skilled / geared enough to drop the boss with 25 people, congratulations - you win shiny purplez! If you're only good enough to do it with 26, tough luck - you get nothing.
That's led to a high-pressure environment; you have to justify your place in that 25. That brings further non-linearity: if you're the 26th best player, you get nothing whilst the 25th best is showered in riches. It's also a pain for guild officers, as someone has to make the decision as to who makes the cut for the raid, which isn't always clear. How do you fairly judge soft skills such as teamwork and situational awareness or balance current performance vs potential? There's probably only a marginal difference between number 25 and number 26, so whoever misses out may well feel miffed.
What all this means is that what may start of as a tiny difference in player ability can end up with a huge difference in reward, as the slightly better player gets better gear, which increases the gap further with every raid.
Of course, its very easy to say what's wrong with the way MMOs work; it's much harder to come up with workable alternatives. That's what makes the WAR idea of public quests, as refined by Trion into the rift system, so interesting. This takes completely the opposite approach. Anyone can join in a rift and help out - in fact, it's actively encouraged through the public group system. Because there's no limit on the number of players that can join in (other than frame rate and lag, anyway!), there is no real reason to exclude someone - even lousy DPS is better than none. There is no issue with "seeing the content" in Rift, as even ultra casuals can drop in on a rift if they happen to be in area.
Under this system, the problem changes from "who gets to participate?" to "who gets what reward?". And that's where things start to get sticky: how exactly do you measure contributions so that skill and effort is fairly rewarded?
It's this issue that tripped up Mythic early on, with some people concerned that the rewards seemed random. What makes measuring contribution hard is the question of what gets measured. Some things seem easy enough, like damage or healing done, but how do you measure debuffing or tanking? Trion talk nebulously about measuring "activity", but it's hard to see what this means other than buttons pressed: it doesn't measure actual usefulness.
This leads to what economists call perverse incentives. People get rewarded more for silly things like standing there spamming insta-cast buffs on themselves than actually contributing to the fight. This problem doesn't happen in WOW because the people rating your performance are well, people, rather than an algorithm which can be gamed.
Long term, this may be the greatest threat to Rift - it's hard to imagine any contribution rating algorithm defeating the theorycrafters for more than a few weeks. It's a noble effort, though, and I hope I'm proved wrong.