Is there a way to turn on my signature automatically? That is, can the
Show Signature: include your profile signature. Only registered users may have signatures.
checkbox be turned on by default? (I keep forgetting to include my sig.)
I think it has been disabled by the admins; I believe I am right in saying that it is considered something between unneccessary and bad form to include your signature more than once per thread/page.
I would submit that the “Use ‘Email Notification’ by default?” checkbox takes up a lot more resources than an equivalent “Include signature by default?” checkbox would.
On a similar note: I use the User CP button to keep track of the threads I am participating in. By default, it always shows me the threads for the past day, even though I would rather have it show me the threads for the past 5 days. Is this possible?
Remembered my sig this time 
This is probably true, but the difference is, of course, that email notification is useful and productive, and signatures are not. 
And (correct me if I’m wrong) I don’t think anybody posting regularly would find the email thing very convenient; constantly getting an inbox full of update messages.