It's been bugging me for years, but I don't really enjoy modern HTML/CSS for fluid layouts when it comes to forms.

To do really nice form layouts in a desktop application, alignment and sizing of labels across the entire form becomes really important to me. Products like Developer Express that have a dedicated form layout work so well for this. And some of the swing layout managers I've used (I can't recall the name, too long ago) make complicated form layouts much more sane than grid, flex and other css layouts.