This is pretty much exactly the game I wanted to make for LD50, but couldn't quite manage! It's great to see that somebody else not only ran with the whole "drag stuff around to form barricades" idea but managed to execute it well. I ultimately ended up doing something similar (still avoiding a horde of enemies, still primarily by dragging obstacles) but much less strategic. I like how much time this one gives you to to try and seal off the entrances: it's much more about crafting a solid barricade and much less about frantically running around away from enemies (though I did end up doing that towards the end).
There's really not a whole lot I'd like to see done differently. Other people have mentioned wanting to be able to push the blocks, but I quite like that having to leave space for your character on the "pull" side prevents you from forming perfectly neat barriers. Okay, it doesn't make a lot of sense but it strikes me as being a little like the convention that tower defence games will never allow you to block off every enemy path: I buy it as a game mechanic even if it strains suspension of disbelief in terms of the physics involved. Having draggable objects (tables, boxes, whatever) rather than just coloured blocks would also be nice, but in 48 hours I think the game is the most important thing and you've really nailed that.
My only real complaint is that after a couple of upgrades, the distance at which you have to pull a block seems to become significantly greater than the distance at which you can attach the rope. Dragging something, having it disconnect, and then having to run back towards it to have another go becomes a little frustrating, especially if the aliens are already pouring into the room. Even that's kind of a minor quibble, though: I think this is my favourite game from the jam so far!