I can't stand it: that a web site has the ability to display a modal prompt sheet that I have to cancel.
It's a feature some people like and use. I like getting notifications from some services I use without needing to keep a browser window open on that page to get notified. And I like not having to download and install yet another flipping app to get that feature.
Um. Email?
How is it hard to imagine a service I don't frequent use but when I do, I want notifications? I use these services a few times a year at most. Why would I want to install an app if they have it?