That’s very subjective. Current trends tend to be colorful with lots of space around elements, mostly thanks to touchscreens. If you look at Finder for example, the toolbar has roughly enough space to fit an entire second row of items. The sidebar groups are separated by a full empty line. Toolbar captions are rare, which I think is a mistake. I’ve done that in the past, but my current designs include captions and it makes life so much easier for everybody. Besides being more discoverable, it also helps with tech support and tutorials when you can say “press x” instead of “press the little icon that looks like x.” Most “modern” apps have just one window, aside from things like preferences and about. Again, this trend is thanks to mobile with their small one-thing-at-a-time screen. But it also helps keep the user organized. Windows did this decades ago with their Multiple Document Interface. It’s not necessarily a bad trend.
But overall, I agree with Sam, though I think both design and functionality need to be considered at the same time. Little details such as spacing and colors can definitely be handled later in development, but how a program flows (single window vs multi window, for example) needs to be considered very early in the UI phase.