This question originally appeared on Quora. What does a user experience designer work on?: "Everything that relates to how a user will experience a website, product, or community. Includes interface, design, offline interaction. After all, any service or product can be seen and judged by its user experience."
Answer by Joel Lewenstein, product designer at Quora:
I think about this a lot, how to get from the designer I am now to the designer I want to be. This non-comprehensive list is heavily influenced by the design process at Quora and the immense challenge and education I've received working with Rebekah Cox, Anne Halsall, and David Cole. I try not to focus on...
My Own Experience of the World
I try to understand how other people navigate our messy, confusing world. What decisions do people make about their lives? What are the patterns in these decisions and the interesting exceptions? What values drive these decisions, both the values people know about and the ones they don't? What is someone looking for out of a particular experience? What delights them, frustrates them, intrigues them? Constantly asking these questions helps build my intuition about how the particular product I'm building fits into peoples' lives. 
What Already Exists
I tend to be overly enamored with what is in front of me and already works, to the detriment of what I haven't dreamed up yet. For people who like building, playing with something they've built is tremendously rewarding. Even the simplest jQuery toggle creates a little adrenaline rush for me, despite having done it hundreds of times . I think this boils down to: 'Shit that does shit is cool.' Unfortunately, shit that does shit isn't necessarily the right shit. It's a constant challenge for me not be seduced by the first UI I build, and remember not to focus on...
Anything But The Product Questions
There are countless ways to justify potential new designs. In the context of designing for an interactive web and mobile product, I try and start with the product questions: Does this change further the product's goals? Which one? How? What is the context of this change, within the product itself or the mental model of a person using it? What behavior are we hoping to drive with this change? Is UI the right way to achieve affect this behavior? What incentives will this create? Are they the right ones? What are the second order effects of this change? Should we make this change at all, even if it seems good in isolation? What are the costs of not making this change? Are we willing to tolerate those costs? It's only once these are sufficiently resolved that I'm ready to focus on the interaction, visual and technical questions. 
What I Can Learn On My Own
This might count as meta goal (learning how to learn), but I'm always thinking about how to best hear feedback from those around me. This can take different forms: colleagues at Quora constantly share surprising and challenging opinions, driven by their unique interaction with our product and area of expertise; friends have opinions on how to best suit Quora for their individual needs; everyone has lessons learned from their work and personal lives, which are usually applicable to the problems we're solving at Quora, with sufficient abstraction. There is an incredible source of inspiration that can come from learning the humility and openness to really hear what people say, pushing past that to understand what they're really saying, and hear patterns across lots of (often conflicting) feedback.
 Two nice side benefits of building empathy: I can work on it all the time, by watching people around me at the bank or on the train, and trying to understand what I see; and becoming a more empathetic person has benefits across many aspects of life, notably in my personal life. Empathy has done more for my friendships and relationships than has learning standards-compliant HTML5.
 There's a really interesting discussion here around whether this is a consequence of being a designer who codes. I'm sympathetic to the argument that the reason I hesitate to build new things is the engineering grunt work involved. I'd be curious to hear about other designers' experiences.
 Another way to phrase this might have been "Solve the Right Problems." This applies at a micro level (learning to focus on the features and improvements that will have the highest impact), but more interestingly at the macro level: learning myself and my values enough to work on teams, projects and companies that are building a world which I'd be proud and happy to live in.
More questions on user experience: