It's pretty common to get emails with links like the following:
I'm usually annoyed when I click the link and have to login, especially on mobile devices.
It's beyond me why LinkedIn requires me to login to accept a connection.
Amazon, the tracking number is in the email, why do I have to login to view the tracking information via Amazon's system? This probably is a problem in about every system that sends tracking information.
If you inline the details of my order in the email, why require me to login to get the fancy PDF?
If someone compromised my email they can reset my password. Some could argue I'd know my password was reset, might not matter if the information is that important.
If I don't have to login on a desktop, make sure the same happens for mobile.
Make sure redirects to a "mobile friendly" version of the site don't pooch my ability to follow through on a mobile device (LinkedIn...)
Many of these links include a random identifier and are prefixed with https, not http. I like to think of these as one-time passwords to access my account. If it makes you feel more secure, only allow access to the part of the account pertinent to the link.
Consider expiring links once the action is executed (accepting a friend request).
Consider expiring links after a time period if the information is sensitive (bank statements).
Because these emails are clogging the inboxes of the world, waiting for us to get back to, like we don't have enough of those already :). I'm of the mentality Do it or Defer it. The easier it is to Do the more likely I'll Do it instead of going to the trouble to Defer it.
Seriously consider sending links that don't require logging in, when it makes sense, it'll make everyone's life easier.