Without actually seeing the work, it's hard to give specific feedback.
Design is an iterative process. Designers are never going to hand you a great looking product on the first time around. Also I find there is a sort of meta iteration in design, because very few designers start from scratch. They start by looking a similar popular product. This is why UX Design has fashion cycles just like any other artform.
As a developer I do have a few tips that I try to to use when building UIs:
- Learn what the gold ratio is, and how to use it to layout a view. It's ubiquitous in nature and art. Something deep in our brains can look at two similar images and find that one that follows the golden ratio better without us even consciously knowing. We just know it looks better.
- Use consistent margins and borders, 4px, 6px, 8px, 12px, 16px is a good place to start if you are unsure.
- Pay attention to negative space. Most of the websites you mentioned, and even HN, have a great amount of unused space. This really isn't a waste, it allows users to focus on the remaining used spaced.
- Start simple, and then go simpler. Your favorite software or IDE may be jammed packed with buttons and views, but that is software for a poweruser. It was built up over years with the expectation that a user will invest time into learning the interface, but these views are overwhelming the first time you see them and look cluttered and confusing. Instead focus on pairing down your UI to the minimum number of elements, and then cut even more. I'm talking way down, to the point that user may only have 1 or 2 things to look at and interact with. Your users will tell you want they want from there.
- Read a design book. Graphic Design and UX predates computers and there is a ton of research and material on it. It is a soft science, so much of its bunk, but you can definitely pick up some nice tidbits.
- Learn how to polish a design. Overall your layout might be nice, but little details could ruin it. It sounds like you have somewhat of an eye since you know your designs look bad. This could come down to simple things like consistent sizing, spacing, and alignment.