End Game Content is replayability, whether that comes at replaying the same game again, only harder or in a different way, or it comes from replaying a certain section of the game, like Baal runs, or running a certain game mode or challenges after the "main game" is completed.
They were banking on item farming to be enough endgame. That would be fine if this were no auction house. However, the auction house essentially killed the "point" of Diablo to begin with. Why farm when you can just buy better gear? Especially since the drop rates were nerfed to make you feel more enticed to visit said auction house. The item hunt itself is bland. All Uniques/Legendaries are boring and or weak. Item "upgrades" are usually a very minor increase to one of your stats, and rarely a large DPS boost, or Armor boost.
D3 has no more character progression. You max out your level before "end game" (Inferno), and you gain nothing that improves your character outside of items. You have no alternate modes, like PvP (coming "Soon"(tm) ), or an "endless dungeon". It has minimum randomization within the game itself, be that in the levels, dungeons, monster varieties, mini-quests...Once you've played through the game 3 times and are playing through it the 4th "end-game" time, you've most likely seen most if not all (and that's only with one character!). It has no character "permanency", so replaying it to level up new toons is limited. 10 characters is all you will need, 5 of each class, female and male, hardcore and softcore, and you are done.
The problem is that Inferno was done as part of the game progression. Ie, you go through Normal, Nightmare, Hell, and then you do the "real game" in Inferno. Thus the beginning 3 playthroughs are boring as sin because you are trying to get through them as fast as possible to get to the "real game". Inferno should have been an aside to the main game, like Challenge Modes or PVP. It should also not have been the only thing to do! As well, make the beginning 3 difficulties actually fun and interesting to play, and offer something to do once you've levelled to max level.
So, You need to improve the core game first. Itemization. Give me something to hunt for. Give me items that are worthwhile. Add resistances/damage types/elemental types back into the game, making items more interesting, as well as giving characters more reasons to choose between items other than DPS. Give me Uniques that are interesting. Give me large varieties in items and socketables to make it interesting to hunt for them. You need more than just 4 gems and they need to do more interesting things in each thing you socket. You need jewels back to give more mods to add to items. You need more varieties of Affixes and Suffixes. You need to increase the drop rates of things that are interesting (uniques/sets). You need to give more ways to improve items, and at the same time you need to remove items from the game in order to encourage hunting. This is due to the auction house, which we know will not be removed.
You need to have some sort of destruction. Why not have it cost an item and a gem to add a socket to another item? Why not have it cost progressively more items and gems in order to add more sockets, up to the max number a specific item can hold? Maybe things like a 2-handed sword can get up to 6 or 8 sockets in it, while one handers can only get 3 (encourage 2-handed usage). Why not make rings and amulets more expensive to stick sockets into?
Why not add a use for white items into crafting? Remember searching for that perfect ethereal item with sockets to throw runes into? Or bring imbuing back. Or how about being able to personalize your items so that you can have "Jaroh's Great Axe of Cleverness" or some such, or let us create a set. There's so much more you can do with items that were done.
You need to add some sort of character personalization and progression back to the game. How about allowing us to "socket" a skill, and by sticking a gem into it, you give that skill some modifiers, like increased radius, or multishot, or lifesteal, or crushing blow, or any number of interesting modifiers that you can attach to gems. Using Jewels, you can get people to add random mods to their skills as well. Changing skills destroys the gem/jewel. Maybe you can gain a socket with experience after reaching level 60? Or using the mystic?
You need to let us get powerful. You need to let us be able to kill waves after waves of demons and make it fun to do so, not kite a couple around for minutes until you finally kill one and then do it again, for the rest of the entire endgame! You need to get rid of 1-shots, at least in the extreme amounts they are using. What do we do with quest rewards once we've reached level 60? Make them interesting and tie them to character development. Give us a way to improve followers, skills, abilities, runes for their skills, what have you.
You need more Randomization. In the overland, you need more varieties of monsters and monster attacks in each area. Maybe a pool of available monsters in each area to choose from, so you never know what you are going to face until you are there. You need more randomness in the overland tiles as well. Make the land larger, with more random blocks in them. Add more offshoots. Remember the "you want people to explore every corner of the map because you never know what you will find?" That kind of feeling.
In dungeons, the same thing, but you also need more randomization to the maps themselves, in the number of map tiles as well as the sizes of the maps, and the "mazeness" of them. You need more variety to the scripted events, so that you don't always face 4 skeletons coming out of the ground here, or 3 skeletons jumping through the window there, or 4 zombie halves coming at you from the trees there, or 3 bats from the holes over there. You need this to be a variety, so that you don't know what or how many to expect, or even if some are bosses or not.
You need more random events for every area of the game. You need more random lines from followers (or else just shut them up!). You need more random numbers of enemies in each group, but overall you need much larger packs, so that you can kill mobs of them. You need more "optional bosses" that are not just "normal" enemies, but unique enemies that aren't part of the main progression. The later acts definitely need "more" everything...
Replayability. You need to make the land progressively larger for each difficulty. You need to add in new areas/monsters/quests in each progressive difficulty. Make the areas larger, more variety, deeper dungeons, more offshoots, more miniquests. You need to kill the story. It is terribly written and we don't want to be forced into it anymore. Or improve it. One of the two. The story needs to be sidelined from the game so that "pressing escape" is not necessary. Add more character choices, whether that be more skills to choose from, or making existing ideas more fleshed out (Melee Wizards and Ranged Barbs for example).
You need to add more modes. Add a Challenge Mode. Add a Team Challenge Mode. Add a Time Trial Mode. Add a Descent into Hell/Endless Dungeon Mode. Add Uber Mode. Add PVP Mode. Put these in the same category as Inferno Mode so that people can choose how they want to farm their gear. Get rid of Whimseyshire and add in an area that isn't an insult and outright mock to part of your player-base. Hell add in multiple areas, secrets within secrets.
Make us want to play the damn game. There's your "End Game"