As it stands, I feel that the Sixth Age has two main problems: Choice and quality.
The Sixth Age has been billed as a player-driven conflict from the start. In spite of that, I don't feel like I've done anything. Choices are cosmetic at best, and it's very easy to tell when you've been limited for the sake of future quests. That's not a bad thing for a number of reasons, the problem is that they're advertising the opposite: the choices reflect the heart of the quest's conflict, but a game that spends an hour respecting your decisions, but decides the outcome for you at the last possible moment is a game that has cheated you. Railroading us is preferable to giving us the illusion of choice. If our actions aren't going to mean anything, don't tell us they will. Tell us a good story without branching paths: contrary to popular belief, not every modern western RPG needs to have those, especially if the finished product suffers for it.
As if to contrast this, the few choices we are allowed to make are left up to a vote. World-changing decisions essentially amount to a popularity contest in a game where the community has historically made a point of ignoring the lore at best, and the outcome is usually clear and unchangeable moments after it starts. In the end, it's another choice that doesn't actually give the player a choice.
This leads in to the second main issue: it just isn't well-written. There is no room for interpretation or depth, and it's easy to tell which characters Jagex's writers want you to support: they're polite, reasonable, and right, even when they have no right to be. Their flaws are downplayed or hand-waved away, if they have any major ones at all (see: Zaros, Zamorak). If you're not supposed to like a character, their flaws will be emphasized. They might be rude to the player even when they have no reason to be or have flaws that are very clearly opposed to 21st century morals and beliefs and very strongly emphasized, to the point where you may wonder if they have any redeeming features at all (see: Saradomin, Bandos). Any depth that these characters may have is either fluid or exists only as a promise. Zaros in particular has gone from a mysterious entity devoted to revenge (Temple at Senntisten), to an enigmatic manipulator (Emissaries) to an utterly alien god of logic with a massive flaw (various podcasts and forum posts), to a caring, warm, and completely honest creature (Fate of the Gods).
Simply put, we're being given a rough draft and being told it's the finished product. Each hint betrays a lack of a clear plan, every revision suggests a long-term change, and player choice could derail all of it in an instant if we choose wrong. There's no getting away from the story line now, but it should not continue until they can determine where they want to take it.