By no means saying these are original thoughts, but even if boss/dungeon mechanics aren't as difficult as retail, there are still a number of factors that will make Classic interesting:
1) A lot of Classic dungeons/raids were about the journey to get there --> i.e getting all the keys to 5 man dungeons, Onyxia attunement questline, MC attunement questline, LBRS to make the key to UBRS to BWL attunement, the AQ collection effort, the Argent Dawn grind for Naxx access.
2) The gating inside the dungeons - Aqual/Eternal Quintessence, Onyxia Cloaks
3) A completely different approach to classes that are not homogenized and maximizing their potential in raiding and pvp
4) Gearing (defense caps, resistance gear), mana management and utilizing a spell rank system, dps having to strictly monitor their threat levels
Again not saying it puts Classic on par with Retail with regards to difficulty, but it will be a change of pace for many players and definitely makes gameplay interesting and more worthwhile (imho).