I love face to face meetings! Apart from getting me out of the home office I just like meeting people and going to new places, sometimes you learn a lot as well. At my last client meeting (actually a CMS training day) they provided a full buffet for us which was really nice (and tasty!).
Non paying customers? That affects all businesses so not related to web dev companies. You just have to deal with it like everyone else.
I see demanding projects as a challenge, not just from a technical point of view, but also working with my skills to find the best solution I can for the client. There will always be demanding customers, that's the nature of some people, understanding how people work and behave is part and parcel of any business activity.
My pet hate? Working with so called "web designers" who simply have no clue about UX, accessibility etc. Mostly they are just "graphics" people who can do pretty visuals and don't understand the web...