I have not yet heard anything back. I am so anxious and worried.
I wouldn't start worrying yet... today is only day 9 of the 15-day timeline they gave you (the processing time doesn't include weekends or the Thanksgiving holiday) and other people who have received the 15-day email have had their visas processed by either day 15 or day 16.
what can cause denial?
It depends. A visa refusal could be for something as basic as not including the correct documents (which can be easily fixed by re-applying with the required documents); or if you don't actually meet the maintenance or accommodation requirements; or if you have something like a serious criminal conviction for which the sentence had it occurred in the UK is imprisonment of 12 months or more.
Does denial mean I cannot ever travel to the UK?
No, but you would need to get a visitor visa (or other type of visa) in order to travel to the UK again in the future.
My plane ticket was for 11/30.
They do actually recommend on the UKBA website that you do not book any travel or plane tickets until after you have received the visa. Just in case it takes longer than you were expecting or in case the visa is refused and you are not able to travel at all.
