I play the game, albeit as a very casual player who has arrived since the launch of 'free to play' which introduced me to an amazing MMO that I hadn't really taken a second glance at in the past, but has gorgeous outdoor environments, insanely well-written and deep quest lines, as well as some overall fun solo and group gameplay.
I've not experienced radiance directly, but plenty of people in my guild have been waiting for this happy day to come. As it's been noted in the previous posts, it's a stat that you had on various forms of high level armor, but it wasn't a stat that did -anything- other than, well, give you radiance points. They might as well have called it Grindage, and you needed x amount of Grindage to tackle a raid. It's not like you might miss more, or not do enough damage, you simply couldn't survive the massive debuff.
Part of it goes along with the otherwise clever dread/gloom mechanics that they use in LotRO, showing how you're facing enemies that make you quail and fear for your very life to such a point that they're all but defeating you before you even face them. Trying to work in a fear/sanity sort of effect, but as you can see, it just wasn't implemented correctly here. If you needed a certain radiance level to enter a raid, that's what you focused on, and that's all you cared about, just so you could get into that raid, and it gimped itemization all to hell and back, and was just a really frustrating way of locking the content.
I think they've made a huge step in the right direction, even as someone who might never see a raid that required Radiance. This, as well as other design decisions, show that they're very much still tweaking, tuning and developing LotRO, even after it's been shifted into Free to Play.