As you have mentioned, the clear measurable indicator is conversion, that is, turning website visitors into paying customers (sales).
Depending on the kind of thing you're selling, a sale might be a quick purchase, or, if a high ticket item, there might be a long process.
If the latter, "conversions" can be measurable interactions which convert the visitor into someone who is actively using your site for resarch, and perhaps signing up for newsletters, downloading a whitepaper, or watching a video. You want the site to be useful to the visitor, and to keep the visitor engaged, informed, entertained, whatever... it depends on the type of site.
For certain types of items/services, you might also recommend to the client that retaining a customer is as important as getting a new one, and therefore you'd want to include information, eg, on the use and maintenance of the product. I'm finding that maintaining this post-purchase engagement is a common point of failure on many websites.
The process can be different for every type of site and every type of product. It's becoming clear also, to many marketers, that user-engagement doesn't necessary just begin and end with the website. The site can be tied into a longer chain of user engagement. Telephone sales, prompted by a website, eg, are something you might want to measure. It's not always easy to track everything. Depending on how much time, effort, and money is involved, you might want to get special phone numbers for specific campaigns, offer coupon codes in emails, etc.
There's an area of web marketing known as "attribution marketing" (one of several names that can be used), where you try to determine what parts of your marketing efforts most influenced your sales. A joke connected with this (and this joke came before the web and before SEO), is that a company knows that half the money it spends on marketing is wasted... it just doesn't know which half.
We have a forum here at WebmasterWorld on analytics and metrics, which might be helpful to you in exploring such questions.
[edited by: Robert_Charlton at 8:19 am (utc) on Oct 30, 2015]