Success looks differently for everybody.
Success in today's digital environment should be based largely around analytics - the data. It is no longer "digital marketing", rather simply marketing in a digital world.
The agility of today's tools and platforms combined with the intelligence of analytical tools eliminates most of the guess work when determining what performs best on your website.
I have experienced first hand being part of a large marketing team told what direction to go in simply based on, and I quote, "I feel it would resonate with our guest" from the VP.
To hell with that. Split test. A/B everything. Multivariate test from ad copy to landing page to content to submission forms. Tools like Optimizely make it almost too easy when married to your paid search strategy and ad copy.
Work backwards from your goals.
"Make a million dollars" is a result - not a goal. Increasing traffic to your site can be a goal. Improving awareness and reach and share of voice, share of market, etc... can be goals.
Working backwards simply means determining which goals are best aligned to which campaigns. All campaigns are not alike and therefore you should not use the same measuring stick when determining effectiveness. "ROAS" (return on ad spend) is a highly malleable and likely the most abused metric I have seen in my career. There should not be one metric to rule them all, not even visits - if the campaign was reach or awareness.
Tailor your success metrics to each campaign using data driven decisions, specifically working backwards from a dedicated goal and you will see your best results yet.