I am sick and tired of seeing Alits all over the place. They were fun and different when we first saw them in Morrowind (TES 3 that is). But now we find that these creatures, who were indigenous to volcanic wastelands, are apparently indigenous to pretty much anywhere. Blackmarsh; okay, it shares a border with Morrowind, some might have wandered south, but the jungles of Valenwood? On the other side of the continent? So wildly different from Stonefalls? Or Khenarthi's Roost; an isolated island by the southwest tip of the continent, about as far from Vvardenfell as you can get? Sure! got wild alits en masse. Northern Elsweyr? Sure, it's just a new Chapter zone, people won't expect new, different, interesting creatures.
And they also put welvas on Summerset, and the legendary Wamasu lizards of Blackmarsh in the deserts of upper Craglorn, and stuck a few in Valenwood jungles for good measure.
Speaking of Valenwood, you know they have wooly mammoths there? Exact copies of the ones in Skyrim? Why not copy-paste the model; skin it white/grey with wrinky skin and call it elephant instead, and you're done! Animal diversity right there. No need to use the same creatures over and over and over again for every zone.
Another gross offender is "trolls" - which appear not only in most of the base game, but also got put into several DLC. Sometimes it's okay; putting them in Wrothgarr was cool, because it neighbours Skyrim, and its homeland of the orcs and goblin-ken and all. But Southern Elsweyr and Vvardenfell? Why? And those zones actually have fairly interesting biomes (giant armadillos and wierd terror birds; shroom beetles and fetcherflies, nix'thingies, etc) - but ZOS couldn't resist throwing a few trolls in there as well... Because who wouldn't want Elsweyr and Morrowind to remind them of Skyrim and Cyrodiil... ?
And when new, interesting creatures and species are introduced (such as Haj-mota) they invariably get reused; often halfway across the continent.

And if this was only an issue among overland creatures I might forgive it, but it pervades Dungeons as well. How many times have you fought a Bone Colossus (big flaming skeleton thing) as a boss? Many Ebonheart Pact players first saw one as Baldreth in Stonefalls, but they pop up everywhere; as a boss in Vaults of Madness, or twice over in Crypt of Hearts (Death's Leviathan in I, and Llambris Amalgam in II); How often have you fought a Harvester with the same old mechanics? Or a Lich using the same set of spells - Crypt of Hearts I, Vaults of Madness, Wayrest Sewers II, Fang Lair are just some of the places with Lich Bosses. They're everywhere.
In fact a lot of "villains" and daedric enemies just cycle through the same handful of characters. I don't want to be all negative; so I'll say that the Daedric Titan is easily one of my favourite monster models in any video game. They're big, scary, and draconic without simply being dragons. But like so many other otherwisely awesome creatures, you get tired of them when they're made delve bosses, dungeon bosses, main quest bosses, dolmen bosses, and so on and so on. The first time I saw a daedric titan I was staggered by how awesome they looked, swooping in from the skies. Now it's a yawn.
Speaking of daedra, it seems the Princes have some sort of diversity hire plan going on, because they keep needlessly sharing minions. Golden Saints, which were always associated with Sheogorath and the Shivering Isle, apparently also serve Meridia along the Aurorans (I assume some developer pointed out they both wear golden armor, and thus must both serve Meridia). Likewise Dark Seducers - again, homed in the Shivering Isle - also serve Molag Bal, because his army wears black. In one of the Anchor fights, Molag Bal will explain that "most dremora serve Dagon", but Bal clearly shows his dremora-based army for the rest of the game. And he isn't afraid of using a few Bone Colossi or Liches either... And it's not that ZOS are incapable of creating unique daedra - just think of Shrikes and those creepy raven monsters from the Evergloam - but they usually don't.

In a nutshell (and I promise this will be the meanest comment I make); it seems that to ZOS taking an Elder Scrolls game across the entire continent, opening up all of Tamriel for us to explore became not an opportunity to let their imagination loose and freely create, but rather an excercise in how much copy-paste they could get away with.

I'm not saying the game is devoid of entertaining PVE experiences - As mentioned I was very much entertained with Za'ji and Caska from Dragonhold, and while the Psijic skill line was a slog, it was somewhat alleviated by the Augur of the Obscure (Thank you again, Ben Driskin, and whoever wrote the lines). I also enjoyed the Clockwork City - mostly because of the conversation with Sotha Sil towards the end, which was amazing (lorewise and all that stuff).
In fact, the main quest lines for each zone are often a good run, but, as mentioned, they offer no real difficulty (any "bosses" have irrelevant mechanics and die in about ten seconds). It's mostly that the zones are full of quest areas that... aren't exactly riveting. And so damnably often it follows the same formula, where as you approach an area an NPC runs up to you shouting a variation of: "Hold traveler! Speak to me before entering; I'm wounded, please help!; No, do not go this way. There's danger up ahead; Ahh, I've been waiting for someone to come by, etc. etc." And it gets rather annoying. In previous Elder Scrolls titles you might happen upon quests in town that would take you around. Or you'd go actively seeking work in guilds, taverns, etc. ESO tries to force a quest down your throat every time you pass by a bit of rubble.

Even in Coldharbour, which is pretty much the pinnacle of the base game (story wise), we're following the same old routines. An alien, daedric plane to explore, and instead of originality we're treated to another trying-too-hard-to-be-funny bout of Lady Clarissa Laurent and her manservant, or helping the Vanos siblings explore another ayleid ruin - pretty much exactly what we did back in Tamriel, but just truncated to a gloomier setting.
And it annoys me because Coldharbour has so much potential. Look at what TES community members, such as Vicn of Nexusmods did with the concept. A Skyrim modder created a more intriguing, exciting and full version of Coldharbour by themselves as a Skyrim mod than ZOS' whole team did in ESO (okay, that was a mean comment as well. Sorry; but seriously, if you play Skyrim check out the mod here; it's staggeringly huge, complex, and captivating).
But ESO offers little of what Coldharbour could've been; take for instance the story of the Daedric Titans. They were created after Molag Bal lured a dragon, Boziikkodstrun, to Coldharbour; tore his soul out, and used the skeleton to create a daedric titan. Why not have a quest where we help Boziikkodstrun's soul escape to Aetherius/Akatosh? You could even make it a quest line where you collect his fragmented soul from various locations - maybe this is hubris, but I think it would make for a more compelling story than helping Stibbons escape a lewd Winged Twilight.

In fact, I think the game could do with fewer quests; but just make individual quests longer chains and award more experience, perhaps with a few worthwhile items put in. Because right now the most efficient way to level is the mind-bogglingly boring Dolmen farming in Alik'r; it'd be nice to have an MMO which actually awarded playing through all these quests and stories.