I would say it's loading time. No matter how good your content is, doesn't matter if visitors don't stay around long enough to read it.
Next would be content.
Yes, I agree that is your content is that good, the layout or look of the site should not matter that much. But... how it looks good or bad, can make an impression, negative or positive, on your visitors.
Guess then domain name. If you want people to remember it or be able to type it in later.
SEO only if that is a priority. But again, with out the items above, it might not matter anyways.
I think all of them are important part of a website. Content is a soul of a website. Page loading speed is an important part of your website. No body likes website which loads very slowly. A good template or theme can make your website looks beautiful and elegant which will attract visitors. So, Overall every factor is work simultaneously to make a website perfect.
The main important part of a website is a loading time and then User friendly URL or Domain Name. To get good rank from google, it is important for a website, it must be seo friendly. Unique content can make your website different form others and attractive part of a site is a good template theme... so all the points are important for a good website.
All those parts are important in the website. However, I will categorize them into two groups:-
Group 1) The content, website loading time and domain name are important.
Group 2) Web template, SEO friendly is secondarily important.
The content is always a king and without an awesome content, you can't drive the traffic to your website. The web template is something which you can change anytime to give an appropriate look to your website.
If you want to create a good and presentable website for me all of them is very important, but if you wan't to increase traffic or collect leads for some reasons I think you should work really hard to produce your best content. Because even though your website is not as perfect as the other, people will spend their time just to see or read if you have a good articles or they can benefit during their visit on your website. Just like vacation or cross-country travel just to find a good resorts even though its a long way journey as long as you know that its a beautiful place it gives you encouragement.
I would have to say that that would be design and content.
When you have new people visiting your website for the first time, you want to make sure they are not having problems to explore everything your website can offer.
Making simple tabs, things that are easy to find and read are among first things to keep a newcomer on website for a while.
Also, the design can be of great importance.
People are picky and pay attention to how is something done as they tend to perceive it like their own and would like for it to be the best.
If you want to attract new people, go with content and design.