The plot's only simple if you haven't played the first two. The water graphics in this game were probably my favourite thing about it ( I totally agree the graphics were awesome). Corypheus is really just a minor character if you think about it. The real plot is about the Elves and only makes sense if you've actually read all the codexes or played DA:O and DA:II. Also, the character depth, build and relationships in all three games is really superb. Probably why I enjoy the series so much. The banter while you're out on missions is great too. Cullen, for example, we meet in the first game and you honestly would recognize him as the same person. His character development over the 10 years since the 5th blight is just amazing.
Inquisition was pretty alright as a game but the strategics and combat options were way too simplistic imo. Like mostly all I did was have to hold a button down to hack an enemy away. Too easy. I honestly didn't even feel much of a difference between normal and nightmare except when I decided to do quests 4-5 levels above the suggested level (as is pretty much normal with my style of gameplay; I like to do the impossible ;P). Nothing beats Origins though and DA:II had too many of the same dungeons for the gameplay to really be that enjoyable although the story itself was pretty alright and of course I loved the party characters.
Im glad you enjoyed your first foray into the world of DA though. ^.^ I actually love this game series so much I bought it for Sean and made him play it and now he's almost as addicted to the series as I am O_O
You should read more of the codexes and lore online from the older games, the thought that went into the world of Thedas is just absolutely phenomenal.