2) In most Mac applications, Cmd-N opens a new something. In iJournal, it's natural for the user to hit Cmd-N to open a new post. And if it's what the user expects, then it ought to work. Cmd-N is currently unused and could easily be made an alternate binding for "Open Window".
3) The confirmation sheet is poorly worded. It says "the post was added to your LiveJournal". But if you're posting to a community, it wasn't added to your journal - it was posted to the community journal. I did a double-take when I first saw this, thinking maybe I'd forgotten to set which journal I was posting to. When the user is posting to a community, "The post was added to the foobar community journal." would be better wording.
4) However, IMHO the sheet should be removed altogether. It's intrusive and accomplishes nothing beyond forcing the user to click OK before doing something else. Simply unghosting the buttons and putting "Post successful" in the status line would be adequate user feedback, and it would make the UI cleaner and more usable.