Count of keywords in meta tag Description is important by 5.4%. More keywords is good. But you should think about right lenght, it is more than 100 and less than 160 characters.
Many pages haven’t any meta description tag, but I count it as lenght zero in my Google results. As you see, top pages often has this value equal to zero, so this results are not perfect. This is all about big javascript code and about some redirection. But I recommend you anyway to have on every your page description meta tag.
Meta description lenght zero or not included in html
In this tag you should have at least one of your keyword, but it is not so much important like some other things. Always remember, that users will see in Google your text in description. If it isn’t human-readable, user just won’t visit you.
Meta tag description with 0 keywords
Meta tag description with 1 and more keywords
Optimal lenght of this tag is 100 – 160. Use some tool, that shows you page title and description as one Google result. If your description text will be too long, Google will cut it. If your text will be short, it will be filled with some text from your page. And I must say this kind of text looks like random to me.
And you have there some another nice graphs to show description text lenght vs position:
Description lenght vs Google
Description lenght vs Google (without top10)