For visual designers talking about accessibility is like talking about death at a birth celebration, which in a way is very sad. At the same time accessibility is seen as a barrier in the path of creativity, which is why we can see many sites with poor contrast and is sometimes hard to read things, but it looks sexy. For me, a good UI must be usable, must offer e pleasant experience, and this is actually the foundation of UX (user experience) but unfortunately today what was called few years ago web designer or front-end designer it is cold UX designer, so actually a visual designer. This is what is expected from a UX designer - push pixels around and inside the screen, paint nicely and that's it.
Also having a true UX position in your company does not mean that a UX will do all of the above, he has just a bit of knowledge from all fields but focuses on the usability and how the users should experience your product - this dude or dudette will maybe not be able to design or code but will know the power of user researches, user journeys, what should be where to make the best use out of it and so on. Again there is no explicit requirement on .... ACCESSIBILITY - which is what they should do especially on usability and user testing side.
Accessibility is no dark magic, it just helps with what the name says, it makes the product accessible for all or almost all people. Accessibility should not be seen as a brake, but as a chance to do something better, throwing two colors one near the other can do everyone, making them look good maybe not everyone, making them accessible is another story.
Accessibility is much more than the right contrast ratio between two colors and no one says that for UIs now we should do only high contrast schemes to be accessible but there are some features maybe you give a chance in your next projects.
Text for everyone
Sometimes designing things which look good on a screen with sexy fonts takes out on the side of using smaller font sizes, which the young eyes can see and read easily, but, depending on your product, maybe your users are stuck with using your design 8 hours per day. After few hours even the younger user starts to strain their eyes to read. How about the users over 50? How about the users with slight visual disabilities? How about the ones with thick glasses? Should they enjoy working with your product?
Sometimes designers get so fond of their 12px light weight font that even the thought of using the same font, 12px but regular weight makes them go bananas. Let them work for 8 hours with their screens and maybe they will get your point, but this is not a solution. If the design team is such a sensitive rockstar so it does not accept any common sense regarding accessibility maybe you should send them home .... or build one accessibility feature where the users can increase the font sizes, change font weight. In this way you keep their precious design but you offer usability for all types of users.
Colors for the people
A large part of the population suffers from some degree of color blindness, as each individual perceives anyway a bit different the colors, what I see is not what you see when comes to colors. But even if this should be basic common knowledge there is always that designer who thinks that sees the right way of how colors should be used and that every user is like him / her, likes the same things and behaves in a similar way.
The sad truth is that 99% of the times the user does not give a shit on the colors as long as they do not disturb the user from performing the task, or they really hurt the eye. Maybe is important at the first use, to give a coolness factor, but as the days go by the user starts to ignore the coolness factor of the perfect color scheme. So why not work with some safe colors, safe contrast ratios? I am not saying that now you should take all visual impairments and try to make one theme fits them all. But if you consider the safe margins for a relative good vision, you still have a usable color palette which gives you a good flexibility in choices, and you add just one high contrast option as feature. You are more or less covered but still free in creativity.
Contrast for everyone
Contrast is maybe one of the most interesting things, since it can fluctuate during the day, one day a certain contrast can be ok for the user, but next day a higher or lower contrast could be better. This topic is a brain scratcher and to be able to provide different contrast levels it already poses a huge development investment since you need to do a lot of math behind, but the easiest trick is to offer different color schemes, and you can easily cover the contrast issues.
Embrace the sausage fingers
One issue for the mobile sites / apps is that sometimes it is hard to touch a button, especially when you have two buttons side by side and the user has sausage fingers. For once, make that button a bit bigger, make the touch area larger so the lumberjack can use it too, not call the kids to manage to touch that tiny area.
Do not ditch the focus indicator especially if your users should fill forms or they can use the keyboard to navigate. I heard at some point that is so 90's to have focus indicators or to use the keyboard to navigate through a form, this is so false! People still use the <TAB> key to move through the fields of a form, especially when they need to type a lot. But even for the hip user who navigates with the mouse, if the user is interrupted at some point the focus indicator is much more easy to identify than to search where the type cursor is blinking in a huge form.
Consider the blind
Is too much to ask to take care that the elements you have on your UI can be seen by a screen reader? We do not live anymore in the ages where a blind person was considered a weight for the society, someone who should stay and wait to die, some sort of satan's work or the punishment for parent's sins. Today they are active members of our society and they should be allowed to enjoy the same things we enjoy. And this one is actually the most easy thing to implement. Screen readers do all the heavy lifting, we should just take care that what we produce is compatible for the screen reader.
Design for everyone
Although it can seem like a Twitter slogan is actually what you should keep in mind. Make your design, be it product design or print or web, accessible for all user groups - young, old, perfect vision, not so perfect vision, blind, regular user, power user, and you can add here many more user groups descriptions. The thing is that I know, design is like football and politics - everyone has an opinion about, has strong feelings about and should not be argued over alcohol consumption. From the doorman to the CEO everyone has an opinion and knows how to do it better, and designers are the worst when comes to this. Put three designers in a room and give them a task, they will argue for hours and in the end the winning side is just the one who is more combative and less flexible, not because of real arguments but just because he sees things that should be a certain way without any compromise.
BUT in the end the ultimate target should be the user, and not your ideal user, but the real users out there. From my point, I know that you can't design to satisfy everyone, but as long as there are no complaints about usability, everyone can read, can see, can use, and they are not disturbed by the graphics while doing their tasks you, as a designer, achieved the "design for everyone" goal. Let your ego aside and be user oriented, embrace the buzzword "user centered design".
Mainly these are the most overlooked aspects, so if you would think about them in the next design, it will be already a step forward.