I found some odd things in The Midden while doing the Good Intentions Quest for the College of Winterhold:

-- what looks like an altar with the Oblivion symbol on the top
-- what looks like an altar with a hand with the Oblivion symbol on the palm
-- a one-way exit leading (eventually) to a crashed "viking" ship with some nice loot ... nice but nothing spectacular

Although I finished the quest, I'm wondering whether I might have broken some other quest? Or maybe just whether I missed something interesting?

The first thing you mentioned is probably the Atronach Forge. There's a book detailing some things about it and how it can be used nearby.

The second thing you mentioned also has a book nearby that starts a quest and tells you how to use it.

The third thing is seemingly just there for show.

Thanks, Velorien -- I think I'll revisit The Midden now!