An introduction to Radiance:
Shortly after it was introduced, Radiance was a number stat attached to select pieces of armour. This stat did two things:
- It counteracted "Gloom", which is a general debuff that all characters who entered certain raid instances and/or fought certain bosses experienced. Too much Gloom and not enough Radiance and your character essentially played as if they lost experience levels (up to, I believe, the equivalent of -4 levels.. or the equivalent of a non expiring death penalty). If you went even further below the Gloom threshold your character would stand in place cowering and would only be sporadically controllable. Beating these instances at -4 levels was generally very tough but doable, especially if only a couple members of your group were that low. Beating anything while cowering was, as I'm sure you can imagine, impossible.
- It gave you a "Hope" buff everywhere in the game world. Hope increases all of your stats (in a sense, the opposite of a death penalty to the point where you are, in essence, gaining levels over the level cap). As I'm sure you can imagine, anyone who did not have Radiance was exponentially weaker than those who did, making the game too easy for Rad haves and overly tough for Rad have nots.
Eventually, the LOTRO devs decided to remove the second aspect of Radiance and left it as what amounted to a hard gating mechanism in a game that had never had any sort of gating mechanism. All of the raids that existed before Moria (and the introduction of Rad) could be entered by any player of any level. Sure they would likely be instantly squashed if they were too weak, but the only gate that existed was one of decent gear, appropriate XP level and your ability to learn the instance.
The acquisition of Rad gear, both in it's early and later iterations, required repeatedly grinding non Rad requiring instances so that you could collect the associated piece of Rad gear. This gave you access to "level 2" set of instances that did require some level of Rad to even have a shot of success at.. and you had to grind these repeatedly until you had their Rad reward gear.. at which point you could finally enter the "level 3" instance.
It was claimed by some that this tiering process was a good thing as it forced players to get some competence at high level grouping in certain challenges before being allowed to move onto the next tier. It was still a huge pain but at least it made sense.. until changes were made such that anyone could just grind the same easy and fast non rad instances by keyboard face rolling to acquire enough Rad to advance. Thus totally negating Rad's one saving grace as a sort of competency gauge.
On top of all of this is, potentially, the biggest issue with Radiance: It and it alone is the only way you can even enter high level raids. Any other armour choice in the game was rendered completely useless by this need to have a single specific set of arbitrary gear. In one fell swoop, high level itemization in LOTRO was virtually killed by a single mechanic.
TLDR: Radiance is a hard gating mechanism that forced repetitive grinding where there had never been forced repetitive grinding before. It also killed itemization at higher levels because gear with Radiance on it was literally the only gear you could wear in higher level instances/raids.
Hope that helps. If anyone else has further questions, feel free to ask.