Q:  When is a 100% complete LinkedIn profile not 100% complete?
A:  When LinkedIn decides to change the criteria.
In February, LinkedIn changed its criteria for a 100% complete profile, and get this--they applied the criteria retroactively! So I know some of you went from thinking you had it all done to not done at all overnight. Oh, well. As in all things in life, we adjust, regroup, and move on.
So, what has changed? Gone are the need for recommendations, summary, and specialties, and in are the need to add specific skills and more than 50 first-level connections.  
I am going to refrain from commenting on what I think about the changes and move on to why this is important to you and how you can get this done.
Why is a 100% complete profile, as defined by LinkedIn, important?
Simply put, LinkedIn has shared their research with us on this point and state that users with complete profiles are 40 times more likely to receive opportunities through LinkedIn. black infographic 
I would call that a big deal for those of us who are using LinkedIn to make something happen in our business world and not simply using it as a replacement for our Rolodex. (Does anyone other than me remember those?)
As important as this is, only about half of the respondents to my latest LinkedIn user survey said they have their profiles complete. I suggest you make this a high priority if your profile is not complete.
How Do You Add Skills?      
Go to the top toolbar, scroll over "Profile," and select "Edit Profile." Once in profile edit, go just below the top blue box, click "Add sections," and select "Skills." 
How Many Skills Should I Add?
You need at least five to have a 100% complete profile, but you can have up to 50. Some of the research I have done suggests that the skills you list on your profile help you rank higher when being searched by that word, so I suggest you list as many as possible. Search ranking is extremely important on a website like LinkedIn.  
How do I get to at least 50 connections?
There are lots of ways (too many for this tip), but be sure to check out Chapter 12 of my book, "There's Gold in Them Thar Hills: Expanding Your Network," for all the ways LinkedIn helps you do just that. 
Simply put, I am pretty sure you have at least 50 friends who are on LinkedIn. Find them by using the search box on the top right. Click their profile, click the big yellow "Connect" button on their profile, write a nice note, and the invitation is on its way. This step should be a no-brainer.
Check out my article on your connections strategy for more thoughts and insights on this very important part of your LinkedIn experience.  
Here is the complete breakdown of what it takes to have a 100% complete LinkedIn profile.
First job position   20%
Confirm first job position   5%
Describe your current position   5% 
Industry and postal code   5%
Second job position   20%
Add 5 Skills   5%
Add your education   25%
Add a picture   5%
Add at least 50 connections   10%