If you are going to collect emails to promote your app launch, send out a useful email to your list every week or two. Your open rates and click through rates will be MUCH higher on launch.
Also, if you are "too busy" to communicate with your customers before launch, you probably won't have many customers after launch.
#5: Writing large blocks of text using a tiny gray sans serif font and expecting people to read it.
Sorry, my eyes just won't hack that this early in the morning.
5. Saying "Done" in the title instead of "Made".
Beta invitation and getting subscribers pre-launch is in many cases a flawed strategy. It is really, really hard for an unknown startup (amongst hundreds of other unknown startups) to stay 'top of mind' for more than a week or so.
For the beta and pre-launch startups I sign up for, I remember only half by the time I can register and use the service. I register for about 5% of those I still recognise. For the rest, I simply don't care any more.
Rethink the pre-launch and onboarding strategy. Just because others say you should doesn't mean it will work for you. THINK FOR YOURSELF.
tl;dr
Mistake 1. They have collected beta invitation email addresses but did not send any emails until the launch. Some users forgot about the product when they received the invitation 3-6 months later.
Mistake 2. They used spreadsheets to track user cohorts and that was too time consuming.
Mistake 3. They focused too much on features instead of benefits.
Mistake 4. They weren't able to find ways to get exposure from blogs.