Raided and cleared everything in classic as a mage, the most difficult part of the game back then is for people to understand what gear to use, especially in entry lvl raiding.
Peoples minds were usually set on "it is purple, it must be better" and "it has a higher lvl req= it mus be better", while blizz designed some godawful items in mc/bwl.
Let me start on set items in general (as pointed out previously by a pala) they had useless stats on them, for mage sets, the useless stats were agi, spirit, mp5 and the "epic" sets from raids even had resistances on them, taking away chunks of the items budget. More often then not, random items, possibly from lower lvl dungeons, were better then the raid equivalent.
Mage BiS in mc times: (Any stamina beyond what made you 1 random attack in raids was useless, as the aoes were timed and you got healed up inbetween)
Head: Crimson felt hat (stratholme) used to not have spirit on it and had more int then arcanist
Neck: Choker of the firelord (mc-rag)
Shoulders: either Magister ( blue dungeon set) or kentic amice depending on the value of int versus crit, dont recall which i used back then.
Cloak: Bloodmoon cloak (ubrs, beats)
Chest: robe of the archmage (crafted from a lbrs pattern) this robe was bis until aq40 with t2.5, even then it was used on some fights for the mana regen
Bracers: either strath (unforgiven, random roll with spellpower) or lbrs wyrmlatak
Mainhand: mageblade (yay the first drop-epic on the list) golemag in mc
Offhand: some lvl 47 quest reward from unguro (had highest spellpower on it)
Ranged: lunar wand of ... power (frost in mc) green wand with +23 that school spellpower ("epic" ones had like 10 spellpower) This wand, or rather the fire version, was bis even in naxx
Gloves: blue pvp set (the blue set had a 2part bonus of spellpower, the epic set had stamina as 2part afaik)
Belt: Belt of the archmage (DM north tribute craft), had a few int more then mana igniting cord in mc
Pants: Padres legs (diremaul hunter boss) until you got tier 2 pants of ragnaros
Boots: blue pvp set
Rings: Ring of spell power*2 (random mc bosses)
Trinket 1: Talisman of Emphemeral Power (ToEP) (mc drop), on use +spellpower trinket for burst, Eye of the Beast for stable dps (quest reward for clearing ubrs)
Trinket 2: Briarwood reed (drop of a rare spawn in ubrs)
so the entire bis included 5 items from a raid, 6 if you include duplicates, and 0 i repeat 0 items from the instances main tier (rag dropped t2 pants, but mc was a t1 instance)
This trend continued for most of the early instances, so you were bombarded with garbage in raids, because 5 man stuff often beat it, ppl just looked @ color and ilvl and used whatever they felt was more prestigus, instead of min maxing.
This combined with general unawareness of the game mechanics made encounters more challenging to them, as they simply didn't know how to play. Granted by the time some1 was fighting 4hm they would have replaced most of those blues, and the later sets were actually better then, but resistance or general gear requirements withheld ppl from rushing through instrances like we are used to now, as some fights simply were made with months of farming the previous bosses in mind.