Which I don't mind. The voice acting on young Eren became quite annoying.
As for the giants, my hypothesis would be either the humans fucked up some shit with gene experiments (the injection) or an epidemic broke out. No aliens for me. While quite a possible theory, I'd prefer the humans did it to themselves. Adds a more horrific feel to the series.
As for the dreams Eren's been having and deja vu's Mikasa's having, I think something horrifying happened before and both of them had their memories partly replaced/erased. I think they might be even from a previous town where something similar happened or something.
It's save to say the humans had some type of technology before looking at those walls (unless the giants build them) and I think some of that technology rests in that basement Eren has the key to.
All in all, awesome start of the series. Can't wait for the next ep.