I'm one of those people who take long breaks from playing Skyrim, only to feel like Bethesda left something out when I start playing again; however, try as I may, I still can't figure out what it is! It's slowly driving me up the wall. Has anyone else felt like this? What do you think Bethesda forgot?? Just wanted to strike up a discussion.

Depth.

The gameplay mechnanics are simple, almost as barebones as the UI. The stories and characters are simple, one-dimensional and forgettable.

For me, it's greater interaction with NPCs. I'd have loved to see the option to ask people about their town, ask about rumours, and just chat a little. The disposition system is sorely missed as well, even if it was half-baked in previous titles. But above all, I'd love the world to react to you more - although Dragonborn has shown clear improvement in this area.

"Detph" may be, more complex interconection of the quests, but most of the times when I role play, I forget about it.
I agree the mechanics are simple compared to Morrowind (oblivion does not count). Everything else is just amazing.
Fortunately, at last for the mechanics, mods come to the rescue!

No character depth. No interesting NPCs. And very unforgettable NPCs besides Nazeem the all hated, and Serana the Iron Mace's toy um yea..

Quests are simple, slightly rushed, and simply gohere-killthat-fetchthis, very linear, and not very impactful on the world.

Incompleted cities, nuking winterhold, scrapping out the arena, scrapping out 50% of the civil war scenes(e.g. the part where galmar chops off Ondelomar's head)