1. Eric hasn't "left". He's simply not the show runner anymore. He's still a producer and still approves of things, just not at the capacity that he did before. I'm sick of people talking as if he's no longer involved in the show at all.
2. You've got to expect the show to be a little different. They finished a huge arc last season and are starting in a new direction.
3. It's only the first episode. You can't expect everything to be all neatly laid out in one 42 minute episode. A lot of things need to build up over time.
4. People change, so do TV characters. Look at what all the boys have been through. Sam's been to Hell, Dean's lived for a year without him. You can't expect them to be the same people we saw at the end of Season 5.