There are a lot of harnesses that start out comfortable but start to chafe my hips on Day 3 of a big wall. The most comfortable harnesses have wide waist and hip belts that are lined with fuzzy material. They are soft around the edges so they won't cut into your hips. In general, the wider the padding, the more comfortable the harness. However, if a harness does not fit right, it will ride up at awkward angles and cut into your side. Try and hang in harness in the store. You will know right away if it is fitting right or not. Keep in mind there are tradeoffs for comfort: the most comfortable harnesses generally don't breathe well and are clunky to free climb in.
This comes down to personal preference. I prefer just one buckle on the waist but many people like having two buckles so that the belay loop can always be perfectly centered. Also, two buckles gives you a wider range of sizing options. While I prefer a big wall harness with fixed leg loops, all big wall harnesses currently come with adjustable leg loops. If possible, get a harness where the extra leg loop and waist belt material tucks away cleanly.
The most comfortable harnesses to sleep in allow you to remove the leg loops and then don't have too much hanging out on the sides. That said, I prefer to take my harness off completely when sleeping and use a two-inch swami belt (or thinner). So it comes down to your sleeping style.
The best gear loops are easy to clip and durable. On a big wall not only do you have a lot clipped to the side of your harness, often it is absolutely crucial not to drop that stuff. For example, you may need to clip the portaledge to your harness while transferring it from under the haul bag to the anchor. Or you might have your ascenders clipped to your harness and you don't want them coming off in a squeeze chimney. Some big harnesses have tons of gear loops at two separate heights. I prefer just one set of gear loops. If you have two tiers of them, the bottom tier is pretty hard to access if your have lots of stuff clipped to the top tier. We also prefer gear loops with some structure so they stick out from the harness. If the gear loops lay flat against the harness, it is harder to clip and unclip stuff.
One some walls, you might free climb just a handful of pitches. In this case, how a harness lets you free climb doesn't really matter. However, on many walls like South Face of Washington Column or The Nose, there are stacks of free climbing pitches. Remember, free climbing pitches on a big wall always feel harder: 5.9 feels like 5.10. This is party due to fatigue and exposure. But it is also because you are more weighed down. You will already have a haul line and ascenders clipped to your harness as well as a big rack. Adding a bulky and heavy harness to the equation makes free climbing even harder. So here you really have to make a tradeoff. For a route like The Nose, I almost always climb with a normal free climbing harness. Sure my hips will hurt a little, but I would rather make that trade off than worry about whipping out of the Stovelegs pitches. On a multi-day hard aid route, I want a super comfy harness, damn the free climbing consequences. And then there is that middle ground where a light but comfy harness like the Petzl Calidris is the way to go.
Check out our complete big wall harness review to see how the top products performed in head to head tests.