You will miss out or in essence be penalized if your site is not displaying well on mobile browsers, beginning on April 21st.
You should make sure your site passes muster here:
Google Mobile Friendly Test
My recent experience: Get good stable traffic on social media for your new site and eventually Google will begin to send your new site (an equivalent amount) traffic - if you are doing social right you should see results after about 6 months. Social is the best source of traffic at this point.
Your site's load times are an increasingly important factor.
That's as much quick and general 2015 SEO advice I can share for free.