Among the biggest complains about current WoW is leveling which is so easy, you just never feel sense of danger. If Blizzard ever addresses this adding optional hard mode, would you choose it over default one?
This is how I see this hard mode: you can active it just like lock XP - speaking with NPC in Stormwind or Ironforge. Then all classic zones 1-60 become much harder, similar to vanilla experience. Common mobs become harder then current elites, elites become 5 times stronger, rares even more deadly and require group. Classic dungeons scale to similar difficulty like hardmode dungeons in Cata. Also all players playing this mode with be merged together from all servers, so it will be more active population.
You dont get any particular rewards for playing in hard mode, but you get achievement for completing leveling in it for every class.