I am not a hiring expert but I think these tips can help someone stand ahead of the crowd. This might also help when you are cold-emailing a recruiter.
Keep the letter to 2 paragraphs. The 1st para highlighting WHY do you want to work there & 2nd para highlighting HOW hiring you is going to benefit them.
Do research about the company. Go through their social, blogs etc. Yes this is the time to be a stalker on internet
Always add a personal touch while explaining why do you want to work with them. This is the time to be as "thoughtful" as you can.
Don't use any copy-paste templates, not worth it. If a company is asking for a cover letter means that they know a résumé has limitations, they want to hear the WHY & HOW of hiring you is going to help them.
Explain something exceptional/cool you did.
This is a progressive post, I will add/delete as I learn. Suggestions welcome