There's a trend amongst modern websites and web apps that is becoming increasingly irksome: hiding the damn "Login" button! If you require me to have an account to use your service (which makes sense), then I don't think it's too much to ask that the actual method of logging in is clear and obvious. However, in reality, a lot of websites (social media and web apps seem to be the worst, particularly those that operate on a semi-freemium model) practically hide the "Login" button or link. Go to Workflowy.com and play "hunt the button" or Evernote.com and realise what you want appears to be fine print!
To be clear, I understand that for most of these services new users are the primary purpose of their home page. Sure enough, in most cases the clearest button or action is "Sign Up". My annoyance is that, actually, the return users are the people that home pages should probably care about. We're the ones that have chosen you as a service, the ones with a history with you and the ones that are most likely to opt in to paid for extras or subscriptions further down the line. High monthly sign up rates look great on paper, but the reality is that without a large, steady user base, no service will last that long.
I also understand that this isn't an intended part of the user interface. Companies aren't actually hiding the "Login" option two or three levels deep; it's usually present on the home page, but I frequently have to hunt for it. Part of this is simply because the "Sign up" option is often too eye-catching, drawing my focus away from everything else and then frustrating me when the option I want isn't clear. Workflowy is a prime suspect. Their homepage draws your eye to the large, centre aligned and bold "Create an Account" action - indeed there's barely anything else on the page! Unfortunately, it means that the "Login" button, which is decently placed, coloured and clearly outlined, is completely outside of my sphere of focus.
All that I ask is that "Login" becomes just as clear and obvious from a user's perspective as "Sign up" certainly already is. Twitter is a great example: both buttons are side by side, with Login actually slightly modified to stand out clearly in a location that a lot of websites use. It's clear, concise and doesn't detract from the options to create an account; it just makes it simple for those long-time users to find what they want, as well.