To "pick one thing" I like most about CS is too tough to worry over so how about my top FIVE...
1. Limitless, easily searchable, quality content from 1600+ builtin sources -and- the ability to add your own -and- Feedly Integration!
2. Each piece of content includes the source, FB and Pinterest Likes, numeric "hot" rating and buttons to either publish or save to Favs to consider later.
3. Generous number of social accounts and blogs you can post to at all membership levels. And "blog" means any blog including your own self hosted with a CS Plugin.
4. Campaign Automation - (this is a monster feature!) schedule posts to all your social accounts in a single weekly sitting (but it really is so interesting you'll want to be more manual!) Think massive post scheduling to dozens of blogs/social accounts. The builtin Planner Calendar is great to view your posting plan from above.
5. Groups - this allows you to segregate (for example) your business Facebook, Twitter, Wordpress Blog etc from your personal accounts and post collectively to each. A truly inspired feature that says the people at ContentStudio think like users.
I mentioned Feedly Integration in #1 but it deserves a second mention because it is the pipe that an ocean of content designed by you flows to your desktop from. An absolutely brilliant inclusion.
Finally - RePlug is fully one-click integrated! Why? Imagine hiding a retargeting pixel from FB, Twitter, LinkedIn etc in EVERY clickable link you post.
Support is all you want it to be!