Normal 5 man dungeons: These should require teamwork (like CC) to clear if you do them with your leveling gear.
Heroic 5 man dungeons: These should require full dungeon gear and good teamwork. (good teamwork = if you mess up one CC or 1-2 dispels/interrupts it causes a very high chance to wipe). Should be very challenging with 5 man dungeon gear even if you do almost everything right. TBC level of difficulty or close to it, plenty of wiping required to learn these for the average player.
Raid difficulty: In my opinion if the average raiding guild can kill one NEW boss every day or every 2 days then the instance is way too easy. It should take from a few days to a week to learn one new boss and maybe even longer than that in rare cases (like the end boss of the expansion). Think about it, it's not like they are giving you a new raid instance every week. If it only takes 1-3 weeks to clear the instance for the first time it will just be boring farm content from there on. There should also always be bosses or a raid tier that is too hard for an average raiding guild to clear. It's fun to always have a new, harder level waiting for you to step in after beating the previous level. No more letting every single player experience killing every single boss and then stand in Dalaran for the rest of the week. Nerf the end content at the very end of the expansion so everyone can see what the instances and bosses looked like but not sooner.
Raid structure: Few deep tiers as opposed to many tiers with little content. I'll give some examples: In TBC the first raid tier was bad: It was just Karazhan basically. You had to grind the same place over and over to get to the next tier. The second tier in TBC was perfect: You had SSC, TK, Gruul, Magtheridon, ZA etc. It was great because it had a lot of instances within more or less the same tier. Another bad example is Wotlk, it was first mostly just Naxx, then Ulduar etc. Then finally just ICC. It sucks to have only one or two instances per tier, there should be 3,4 or even 5 different instances per tier. I don't care if that means we only have 2-3 tiers per expansion. It's better to have 2 deep tiers than 5 tiers with only one instance in each.