One of the biggest things I'm seeing right now is the lack of commitment to the on-page efforts.
Titles, tags, metas, header/footer, sitemaps....it can all be very time consuming and overwhelming, but it's crucial. So build your site correctly the first time, and it will save you from having tons of headache in the future.
MOBILE - when is the last time you actually looked at your own site from your phone?
Simple changes can have a profound effect on your CTR's and time on site.
Do this often, as automated site updates can really mess things up.