What makes an online course a valuable learning experience for a student? I just taught my first course that finished last week and I was asked to teach the same course for next semester. The only difference is that this time I will be teaching the course online. 

I have taken online courses before and haven't been super impressed. In my experience the instructor will assign reading to be done within a week and any assignments that will be completed from that week. The instructor doesn't teach in an online environment but more so moderates that way students learn. The student reads the text, does the work. The instructor responds to emails, grades assignments, and moderates discussion boards posted in the online forum for students to collaborate. 

This format has the student teach themselves and from personal experiences students who are not familiar with the course material beforehand generally don't do as well. 

What are methods to make the online learning experience less informal and more of a learning experience?
If your course delivery platform has things like forums or collaborative writing spaces, you should make use of them to get students interacting with each other.  I don't know what discipline you're in, but you can ask students to comment on the readings online, write responses to each other, that kind of thing.

I was teaching a regular course but was ill for a week, and not wanting to miss that whole week of class, I scheduled a live chat during the time the class would ordinarily meet.  I think we had the best discussion of the semester that day -- students who wouldn't ordinarily speak up in class "spoke" during the online chat.

Also, set boundaries for yourself; otherwise you'll find yourself answering emails at midnight in your bunny slippers.  Tell students if they email you after a certain time, they will get a response the next day.  And think about how you feel about responding on the weekend.

What is your discipline?  We might have more tips, depending on what it is.