#1 Finding opportunities to obtain high quality links (.edu, .gov, high PR). Most of the advanced link building work is highly manual and is typically based on email outreach with unpredictable ROI. I pretty much avoid advanced link building tactics and focus on quality content creation and promotion through social networks (this lands be good links naturally).
#2 Old links are constantly expiring at different rates and while there are tools that will show you expiring links after it happens you can not predict a loss spike ahead of time and try and make up for it. Thus, links have to be acquired on a very steady and consistent basis.
Just to clarify - directory submissions, profile creation, commenting on blogs - these tactics I consider basic and not what I am referring to above.
The problem that I see many marketers face is that they think too much about acquiring links rather than earning links, no this might sound hypy however if they start investing in creating link bait campaigns and creating content assets that will attract links, shares and mentions.
This change in mindset will move marketers away from strategies that have short lifespan :)