what do schools look for in a student’s ECs? Do they want a huge list of sports and clubs that you participated in or a few things that were focused on or something else?
Leadership and a focus or theme. Students who post here saying that they have “tons of EC’s” are not going to impress adcoms.
High achievement in an EC is likely a significant factor. For example, winning a state championship in a sport or being a key player on a high school sports team winning a state championship is probably worth more than being a bench warmer on several different high school sports teams that lose most of the time.
For most schools - all but the most selective - ECs are just another way of illustrating who you are, what you care about, and what you have to offer the school community. It’s also a chance to develop skills that aren’t developed in the classroom and show that you have a life and an identify beyond being a student. At the very most selective schools - schools where everyone has top grades and test scores - it’s also one way to set yourself apart from the crowd. For that purpose, you would need to demonstrate an exceptional level of achievement in something. Doing lots of ECs just to fill in all the blanks on the common application (or to look ‘well-rounded’) is counter-productive. Everyone in that group of high achievers is fully capable of signing up for and juggling lots of ECs - many do. It’s not differentiating at all and it detracts from the effort you could have put into the things you really care about: sleep, friends, your real interests, etc…
Leadership, passion, dedication.
Keep in mind that out of the 3,000 or so 4-year colleges in the US EC’s are important for admission at only a small fraction. Perhaps the most selective 100 or so. You can see what factors matter in admission at schools you are considering by looking at their Common Data Set filings. Most students worry needlessly about EC’s when they’ll play only a small factor or not even be considered!
The question about impressive EC’s comes up regularly on the forum. There is a thread with several posts by Northstarmom, a Ivy alum interviewer, about what constitutes impressive ECs from the point of view of the most selective colleges. The post is at http://talk.collegeconfidential.com/what-my-chances/210497-those-ecs-weak-so-what-s-good.html
As you will see from that link, at the most selective colleges they are looking for depth more than just participation. Stanford, for example, says
2 very interesting articles about ECs that stand out and how to get them (same author, different examples) are at [How to Be Impressive](The Art of Activity Innovation: How to Be Impressive Without an Impressive Amount of Work - Cal Newport) and [Save This Grind?](Case Study: How Could We Save This Ridiculously Overloaded Grind? - Cal Newport) I don’t buy into his underlying explanation of why they are impressive, but take a look at these 2 articles and I think you’ll get some original ideas.