Haven't raided since Cata, but here's my opinion.
Back in Vanilla/TBC, raid content needed to be "hard" because that's all there was for us. With the introduction of "Heroic" mode, the need for normal mode to be challenging ceased to exist. People who want "Challenge" can get it in Heroic raid content, people who just want to casually raid with a pre-made group (guild/friends) can raid normal modes, and those who just want to raid on their terms, and not be forced to talk to anyone (aka me these days) have LFR.
What's always annoyed me (more so when I raiding then now) is you get these mouth breathers coming onto the forums and having a big cry that "WOW's too easy". Meanwhile, only Top 100 guilds have actually cleared all the content, and they're still in normal mode "remembering back in the day" when it would take you several weeks to down a new boss. Newsflash, get out of normal modes if you're "too skilled for WOW" and actually play the content that was designed to make WOW "Not easy".
Bit of a rant, but the fact is, the only people who want Normal modes to be challenging are special snowflakes who can't do Heroic content and want to feel special about normal raiding. There is no reason for Normal mode to be so challenging that your 'average' guild can't clear it with minimal effort at a certain ilvl, or "advanced" guilds clearing it with previous Tier gear.