I think the biggest reason a lot of people don't like LDCad's gui is because it doesn't look familiar at all and it looks kinda... old. A lot of desktop software often has a gui that uses either native OS elements (winforms, QT, etc.) or just looks more fleshed out (JavaFX, MS Office Ribbon interface, Autodesk ribbon interface, stud.io). Yours is neither. It doesn't use/look like native OS elements, but it doesn't look particularly well-designed either (no offense 
).
Keeping a self-made gui is certainly possible. You absolutely don't need to go native, just take a look at Adobe software, Autodesk software or even Bricklink's stud.io for example; not native at all but pretty popular and people think it almost as easy to use as LDD.
I'm by far not a designer, but I think improving the looks of your gui will be the biggest improvement for most people, not necessarily its functionality. For example, yours is (currently) very flat and square. Flat is kinda a trend in modern web/app design, but in a different way. Modern design guidelines such as Google's Material Design (which I personally really like) and Microsoft's Metro use color, spacing and (slight) drop-shadows to separate ui elements, whereas you use borders (with pointy 90 degree corners) to separate the ui elements because everything is has the same blue color.
But that's just a single example. I think if you take a look at those design guidelines you might get some ideas on how to improve your ui. And I don't mean quite literally copying one of those designs (although you certainly could
), I mean the thought behind them and why they did certain things.
But again, I'm not a designer, these are just my very personal 2 cents
).Keeping a self-made gui is certainly possible. You absolutely don't need to go native, just take a look at Adobe software, Autodesk software or even Bricklink's stud.io for example; not native at all but pretty popular and people think it almost as easy to use as LDD.
I'm by far not a designer, but I think improving the looks of your gui will be the biggest improvement for most people, not necessarily its functionality. For example, yours is (currently) very flat and square. Flat is kinda a trend in modern web/app design, but in a different way. Modern design guidelines such as Google's Material Design (which I personally really like) and Microsoft's Metro use color, spacing and (slight) drop-shadows to separate ui elements, whereas you use borders (with pointy 90 degree corners) to separate the ui elements because everything is has the same blue color.
But that's just a single example. I think if you take a look at those design guidelines you might get some ideas on how to improve your ui. And I don't mean quite literally copying one of those designs (although you certainly could
), I mean the thought behind them and why they did certain things.But again, I'm not a designer, these are just my very personal 2 cents