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I'm going to point out features that are common among all TES games that I've played (Morrowind, Oblivion, and Skyrim) and compare them to what ESO is offering. At least then we can have a basis for discussion on whether or not ESO is actually a TES game.
TES Games - Allows you to pick all races, either gender, and allows extensive character customization.
ESO - No word yet as far as I know, but I'm sure it'll have an extensive character customization template. You can't pick Imperial. Elves look Tolkien instead of TES Elves.
TES Games - Free form skill-based progression. Specialize in whichever skills you want, can max out all skills if you want, and can make any manner of character you want.
ESO - Class-based system, although loosely class-based. Not sure if you can use any combination of skills that you want. For example, can you use my favorite combination of skills (Heavy Armor, Swords, Destruction Magic, Restoration Magic, Lockpicking, Mercantile, and Speechcraft)?
TES Games - Twitch combat for magic, melee, and ranged. No hotbars. No abilities to use other than power attacks.
ESO - Twitch combat as far as I know. Hotbar. Abilities you can use. Elite ability. In other words, they are melding TES and GW2 combat together.
World Exploration and Interaction
TES Games - You can explore the entire map. You can interact with anything in the world. Dungeons and Buildings are instanced. Your character is neutral and has the option, but isn't forced, to join various guilds and factions.
ESO - You can only explore the lands belonging to the faction you're forced to choose at character creation. The world will be split into zones. The zones will be split up into instances. Dungeons will be instanced. No word as far as I know of buildings being instanced. The commonly known guilds will be in the game and joinable from what I heard. Not sure about the Dark Brotherhood though. Not sure if you can rise to in power in all the various guilds and factions like you can in TES games. You're forced to choose a faction.
Quests and Content
TES Games - You have to interact with NPC's to get quests, there's no icon over their head. Quests aren't linear, nor are they level specific. You don't have to do any quests at all. There are side quests, faction quests, guild quests, and a main storyline. Dungeons are dangerous, dark, keep you on your toes, have traps, and require lockpicking or some magic that can unlock hidden areas and chests.
ESO - No idea if there will be an icon or not above NPC's heads. No idea if quests are level specific and if mobs and etc scale to your level. No idea if quests will be linear or not. There will be public quests like GW2. Dungeons will be public and some will be private ( i think). No idea if they will be run and gun dungeons like in typical themeparks, or if they'll be like TES game dungeons. Will have RvR.
Housing and Mounts
TES Games - You can buy a house and decorate it. You can buy a horse, which can die.
ESO - No housing. No idea about mounts and if they have them, if they can die.
Some core features have either been changed or not included at all. For example, you cannot choose the Imperial race, cannot explore the entire map, cannot have a house, they modified combat to appeal to the GW2 crowd along with questing, class-based instead of skill-based, and zoned off the world and made multiple copies of the same zone.
IMHO, all those things have made ESO more of a MMO than it is a TES game. Hell, it can barely be called a TES now. There's no reason the developers couldn't of taken all the features and game mechanics from TES series and ported them into the MMO environment. There's no rulebook that says MMO's have to have such and such features and be made a certain way. The only rules for a MMO is that they allow you to play with thousands of other people at any given time, and that the world is persistent.
A lot of people are psyched about ESO because it'll have RvR, a throwback to the DAoC days. As an original DAoC player, I would love a DAoC 2 or another original IP that uses that formula, but as a TES fan, I expect a TES MMORPG to be just like all the other TES games I've played.
This is why I personally am not excited to play ESO. In fact, I'm the opposite of excited when it comes to ESO. ESO may turn out to be a good game, but ESO fans cannot ignore the fact that they took all the features and mechanics found in TES series and either changed them or didn't include them in ESO, thus making it NOT a TES series game.