Replacing the T with a space is explicitly allowed by ISO 8601 if context makes it obvious that the date and time belong together. I prefer the version with space for many purposes since it's easier for humans to parse at a glance.
You are correct that time offsets are used instead of timezone names. The most obvious reason for that is that many names are ambigious (EST is either UTC-5 or UTC+2, depending on whether you live around the US or around Egypt).
Point of pedantry: RFC 3339 allows replacement of the T with a space; ISO 8601 merely allows its omission (and that only by mutual agreement of all parties to the information exchange).
You are correct that time offsets are used instead of timezone names. The most obvious reason for that is that many names are ambigious (EST is either UTC-5 or UTC+2, depending on whether you live around the US or around Egypt).