It has great world building. In fact, Ian Frazier and Rolston would often joke in interviews all the time that their world design was so strictly coordinated by RA Salvatore's lore, that they couldn't just add things without them being completely justified by the lore as well as given the okay by RA. His example was that in most RPGs you can just add a tower with an evil wizard there and call it a day. Salvatore wouldn't let them do that and the overall world building and design was treated that way, i.e. with a close eye to detail and making the entire world coherent and in tune with the written lore.
There are several large zones each with their own architecture, terrain style, weather and atmosphere. Plenty of lore to find, books to read, quests and questlines that greatly expand on the Amalur universe and its lore. Each zone and interior "bubble" has its own themes, stories and ideas that are displayed, not only in quests and dialogue but in the design of the "bubble' itself. They are not overly done and fit well within the world as a whole, i.e. they are believable. Note that I use the term "bubble" a lot when describing Reckoning's map, because it is comprised of 5 very large "zones", and each "zone" has several "bubbles" of terrain to explore within that. The bubbles are interconnected and the zones are as well, but you only get a major loading screen when entering a new zone. The bubbles themselves are quite large, usually with several points of interest spread out, and the game never feels claustrophobic because of that. Each bubble has a very strong feeling of being hand-crafted, and they managed to make nearly every single one unique. The overall zones are huge and there is just a ton of exploration, where you find new things and twists all over the map, discover new, often beautiful areas and generally never quite know what to expect in each spot so it keeps you wanting to explore further.
As I've said a bunch, it's like TES in many ways. It's designed to be a game world you can "get lost in", spend hundreds of hours running around, collecting loot, doing quests, talking to people, crafting, following the story, examining the lore and so on. It has different ideas implemented than TES, obviously, such as more of a Diablo-style loot system (with hundreds of unique items as well as 70+ armor sets), a fast-paced action combat system (of course it's still based on your character's skills and abilities, but you have to move well with the controls as well), and a more colorful, cartoon-ish (almost) graphic design. But there are significant story bits in the game as well, and often you will have cutscenes mid-dungeon, at the start and end of the area, as well as many other spots. It focuses more on a cinematic approach than TES does, and IMO probably offers more of this type of storytelling than TES does.
It does have some really standout questlines as well. The factions are all generally good, with the House of Ballads being *really* good. The conclusion and last zone/mission in that game would be the best content in many RPGs. The Legend of Dead Kel DLC is more of an expansion, with at least 20 hours of new area to explore and quest through. I have said this a lot but this DLC is some of the best ever in an RPG. It shows that the devs were really, really starting to find their groove with the game, and I just wonder what could have happened with Reckoning 2. There are also good quests and little stories spread throughout like any TES game, lots of interesting places to explore and vistas to see.
Again, that's not to say it's without problems. Use mods, look into what I've said earlier in the thread and use them accordingly. If you do those things the game will be even better. As it stands it was a very high quality game that I enjoyed a lot and will have to return to sometime. I even played on the Xbox 360, so I'm sure the PC version is superior once you add mods. On the 360 it also stands as one of the better console RPGs out there as well, but that's another topic. Any RPG where you can spend 200+ hours in the world is a rare thing, and the devs in interviews even promoted it as having 300+ hours of content, which I certainly believe, as I played 250 hours or so and didn't see the last 2 entire zones! So, it's huge. Big Huge.
But it's also quality. It has a ridiculous amount of content.