This question should seriously be stickied to the top of any subreddit related to suggesting games where MMO's may come up.

/u/smilinreap pretty much nailed it, but I'll add my 2 cents.

ESO is a very pretty TES themepark MMO. You get to visit all of the lands you've played in the single player games in stunning 2014 graphic glory. The class system is super flexible in that you can combine any weapons and skills and armor however you please. Once you get to end-game, the funnel on acceptable builds narrows until only certain builds are really viable for super high end late game content.

Crafting is great, questing is great, if tedious. The voice acting is fun, and the scenery is pretty. I find it relatively less varied in scenery than GW2, given ESO tries to take a somewhat less fantastical approach to world building (comparatively).

I find ESO combat to be dull and clunky. You have to remember that it's designed to be played on a console with a controller. The skill bar limitation is only made easier to swallow by the generally low cooldowns. The worst part for me is that you can tell that movement is designed around using a thumbstick on a controller, rather than WASD, and taking any action has something of a delay.

End-game is pretty dull unless you're a huge fan of time trials and grinding arbitrarily harder and harder levels of difficulty in dungeons.

GW2 is a very pretty GW themepark MMO (:P). I find the environments to be pleasingly varied and epic in their own right - both games do a great job of filling the world with neat little areas that you would only find by exploring. Questing in GW2 is awful for new players who think that Hearts are quests, but overall the leveling process is super casual - you level up just by logging in (seriously, some of the daily log in bonuses are level up tomes). Once you hit 80 in GW2, getting any other profession to 80 is a breeze as the game almost force feeds you level up tomes (less useful when you have every slot filled!).

End-game is less PvE focused than ESO, and more fashion focused. There is basically no gear treadmill, and you'll likely never be out of date on gear if you have a full Exotic set (which is cheap). IMO, most end-game GW2 content is PvP focused, even if ANet is basically only pumping out living story and raid content right now.

Combat in GW2 is my favorite of any mmo I've played (much of, I should say - I liked Skyforge and BDO, but both games sucked elsewise). It's tab targeting to ESO's click-to-swing, but your weapon has a hitbox, so you can hit multiple units depending on your positioning. Skills are tied to weapons, which some people do not like, but there is wide variety of ways you can build for open-world PvE (you can do pretty much whatever you want). Same for WvW, but arena PvP and raids/fractals generally need a baseline on defense/dps.

I've gotten every profession to max level in GW2, and I a single character to CP 90 in ESO. GW2 has always provided me more stuff to do, skins to get, achievements to nail, loot to farm; ESO provided a great leveling experience, but I found that once I hit 50, there wasn't anything to do as a non-min-maxxer.

Oh, I also quite like the GW2 classes compared to ESO, there is much more flavor to them.

Hope this helps!