"We don’t want to reveal much, other than to say it is an interdimensional being that has more in common with the shark from Jaws than Pennywise from It. When the monster enters our dimension, it’s like a shark breaching the water. Very much like a shark, it drags its prey back into its home, where it feeds. Each time it enters our world, it leaves a small tear, or wound. That’s what Joyce found in her wall in episode 4, and Nancy found in the tree in episode 5. These tears are almost like portholes into the Upside Down. But they don’t last very long. Like wounds, they eventually heal and seal up."
My interpretation from that is Eleven didn't really open the portals but was blaming herself. She made contact with the creature which was then aware of the real world and could come in to hunt. It hadn't come before because, while it seems like it was able to, it had no knowledge of the juicy feeding ground right next to itself. I definitely got the Lost vibes but this show doesn't get as outlandish. The biggest mystery in Lost was how they were going to tie everything up together in the end but that never happened. With Stranger Things, from how much was wrapped up nicely in season 1 and from interviews with the showrunners, it seems like they have a good handle on the mythology of their world.
The similarities with the anime Elfen Lied were so strong that by the end of my binge I was sure they had major influence from it. They've said it was their main influence but, to me, it borders on remake territory except toned down in violence, nudity, and general craziness. There wasn't an entire shadow realm in Elfen Lied but the psychic arms were extradimensional of some kind. Most of the characters have analogues and their motivations and interactions are mostly the same. As a fan of Elfen Lied, this was a really cool show to watch.
