Just getting started, but would appreciate if someone could advise on whether it messes up minor quests when you go in and clear barrows or bandit camps you discover on the way?

No, the only spoily places are locked and you need a key. That said, a lot of ruins have their own cool quests on the entrance or once inside.

No, I've been sent back to the same bandit holds more than once and each time there is a new set of bandits waiting. I've even been sent to kill the same dragon twice, the second time there was a new dragon to replace the one I had killed in the same spot before.

I think there's a couple of places that might mess up more specific quests though like the cave with the pit wolves in the cages. That's from a named quest though not one of the Miscellaneous ones.

There is some spots where...you will have to kill a Bandit Chief...and HALF The time they will put the quest specific guy (Like a Certain Argonian Backstabber) WITH The Bandit Chief..so its kill them all and get two quests done.

I've had one with a specific set of "Rising" Thieves guild where the Bandit Chief isn't there, but my quest specific guy is...

Kind of meh...I'll come back when its repopulated.

It spoiled a quest for me in...

Like everyone else has mentioned, everything respawns for you. I have even had rare/very nice treasures respawn.

The main problem with exploring these areas before the quest is given to you, is if there is a quest item in the area you are exploring and you pick it up...then it is stuck in your inventory until you are given the quest and can complete it. I have quite a few quest items I now have to carry until I can figure out where to get the quest. I have been able to display my quest weapons on my weapon rack though.