Some minor additions:
-It's not just the AI that has issues with being abused by blocking, the AI will unintentionally do it to you too. Along with them requesting you to move, I'd like to request them to move/allow you thru. Haven't you ever had the case of an AI warrior fortifying cause it's too chicken to attack a barb camp nearby and he happens to sit on the only path? Or allied workers blocking your army?
-Not only can you block with cheap units, you can block with workers. Blocking their settler with a warrior makes some sense, if abusable. Blocking a pass with 1-2 workers doesn't. I think the game should allow stacking 1 combat/1 noncombat among friendlies, not just your own units.
edit: oops, tired, just noticed that'd end the warrior block, but not the worker

But allowing 2 noncombat upt allows multiworkers...bleh.
-The pathing AI is not so good, regardless of what it wants to do with the units. I watched a scout on automove go back and forth running into the same mountain on the edge of a civ border for 5 turns before I took it over. It should have either repathed around the civ (there was a visible route) or detected the stall and given me back manual control for new orders.
-It canceling automove when it's target tile 10 turns out has a unit occupy it is sooo annoying. Like another poster noted, I had to give up on giving movements turns out for large groups and just manually move starting from the front every turn.