Hi SF, I think you've got a pretty solid start to the hobby here. From what I can see you've got a grasp of the basics and it's probably a matter of polishing up how you execute those as the next step towards making the army look better.
First up: mold lines - this guy here for example
has one on the side of the helmet that sticks out on reasonably close inspection. Cleaning up every single mold line possible will go a long way towards a professional looking army.
Second: primers, paint thickness and the importance of smooth - again using the guy above you can see where the paint has an uneven thickness to it that makes him look more like a model than a Marine to the viewer. If you're not already using a spray primer or undercoat that's always a good first step to a smooth mini, after that you'll want to look at thinning your paints and improving brush control so you can neaten up your lines and start to work on shading and highlights.
Third: Shading and Highlights are the key to making a scale model look 'real' so time spent here is almost always worth it.
With that purple you could experiment with Leviathan Purple Wash as away to add quick and simple shading to your minis. A black wash for your silver metals and maybe a brown one for your golds will also go a long way to giving them a more 'real' look.
Next, if blacklining is attractive to you (that's where you have all your panel lines and divisors black - for example on the leg of the linked mini) then consider a Micron Pen. I use them for black lining as I love the look myself and they give me better results than a brush most of the time. For highlighting it's a matter of style and taste but working towards smoother changes from dark to light with multiple highlights, the use of washes and other techniques is probably the next step to investigate.
Fourth: the little details - with the basics covered little details like the detail in the backpack inset, details on the BP
like buttons and symbols and so on can really make a difference in making a model 'real'.
All up you're off to a good start and you're in the right place to learn about the tricks and techniques that'll take you to the next level. Make sure to look around the PC&A forums for inspiration and technique and don't be afraid to ask questions when you see something you like - most everyone here will happily explain how they did something in as much detail as you want and many of our best painters have written and even video tutorials you can follow too.