It really truly is impossible and unfair to compare the two. If you scaled up the raids to level 90 they would certainly be easier than LFR. They had less mechanics, easier enrage timers (did they even have enrage timers?), etc. They were also tuned differently. Remember having to chug mana potions? It was a different game back then. Rotations were different (easier), gear was harder to get (3 items for 40 people per fight?), etc.
Also, don't forget all the broken talents and gear (that either didn't work or gave useless stats). Entire specs were written off by serious guilds.
It's silly to compare the two.
Not a chance, if you put in your bog standard LFR group into a vanilla 40 man raid, they wouldn't kill one boss. Remember, back in vanilla we had limited specs, limited spells. If we had the same abilities at our disposal as we do today, then yeah sure, it would probably be fine.
I'm not saying vanilla was hard, but almost anytime I do LFR, everyone is usually below 100k DPS. Sometimes you get a LFR hero who is in heroic gear, but that would not be counted for reverting back to vanilla 40 mans.
Mechanics wise LFR shits all over classic Of course there are a few bosses like Nefarian with the class-calls etc. That was considerably more challenge than the majority of tank n' spank bosses but generally LFR completely shits over most of the classic raid encounters.
Stains on the carpet and stains on the memory
Songs about happiness murmured in dreams
When we both of us knew how the end always is...
I enjoyed it plenty though, especially as I dumped my mana as hard as possible and still stayed ahead on the meters when I FD+drunk, despite being laughed at.
Staying at 41 yards when Lucifron and Gehennas had their curses was also fun. Being hit capped throwing out Tranquilising shots, kiting Razorgore (albeit not MC) et c.
Laughing at other Hunters who insisted on using Hurricane. Using AV-arrows against Ragnaros because the effect gave huuuuge blue arrows sticking out.
Having a raid wipe because a Paladin got the bomb at Baron Geddon, in the midst of a chat message ("Hehe, I bet you will get the bowwwwwwwwwaaawwwwwwawwaaaa11wwww44ww44ww44w111wwwww")
While it may not be to the taste of current day players, it was my taste. It would probably be more difficult to the those doing LFR than LFR was, when I left. If nothing else for the different formats.
LFR is obviously harder because you have to level 90 to kill stuff, even the most hardcore level 60s that farmed Naxx40 wouldn't be able to kill a boss in LFR SoO unless they leveled to 90, end of discussion.
Mods, I have spoken... please lock this thread.