All content is designed for a certain level of gear and skill. Most of us fail at that level of skill, so we compensate with more gear. Or we have inferior computers; framerates can really hurt performance. Whatever it is, for the most part we go into content 'overgeared' and don't think much of it.
Sometimes Blizzard decides a boss is overtuned and nerfs it. We saw this a lot in BC, with many fights receiving major nerfs weeks or months after they were released. And then near the end BT was nerfed heavily, presumably so people could see it before we stopped caring.
I don't mind either of these. Overgearing just seems natural; gear is central to the game, part of the process of gaining strength to take on bigger challenges. A hamster wheel, but that's life. The big nerf, that feels like Blizzard looked at how fights were going and said "oops, too hard." It's a balance decision.
This 5% stacking buff feels more like "Oh you poor slow guilds, here's a cookie. And some unearned damage, healing, and health. Don't forget your homework on the way out!" It feels almost insulting. Overgearing, at least that's something we earned and worked towards.
I'd mind it less if it wasn't on by default. That would allow guilds to individually say "We're not getting anywhere, let's see what this can do for us." Or to say "We almost have this, let's give it a few more weeks."
When it's on by default, it's far harder for a guild to say "We want to refuse this buff to keep trying as we are, and keep wiping." I know that in effect they are the same, but the mind is a weird thing. Who is going to say "Let's make this harder than it has to be"?
Last night we did a lot better on Valithria. How much better? It's hard to say. 5% more healing is easy to calculate (if we assume no one died to lack of healing, which I'm willing to assume to simplify things). 5% more damage? That's harder. Getting suppressors down a little bit sooner can have a big impact on healing; especially if you consider that a suppressor at 4% health is so close to 0%, but is still debuffing her. And if we do take into account player deaths due to lack of healing; consider the blazing skeletons; killing them sooner makes them easier to heal through, as does the buffed healing, which combine to give more time to heal her and more players alive to kill suppressors faster meaning more healing on her... You get the picture.
If you were 1% from beating her, you can say you got 1% better. But what if you're going from 65% to 80%? Clearly that's more than the 5% healing, but 5% damage, 5% health, those multiply the healing. Maybe you only truly improved 65% to 70% and the rest is just a handout.
It's kinda killing ICC for me. The sense of improvement is weakened. Just stand around and it will get easier.
 And as fate would have it, I wrote about overgearing (and difficulty) over a year ago. Though with a different angle. I guess it's a "If you liked this you might also enjoy..."