Yes, it's a simple standard: make sure that you follow the Windows User Experience Interaction Guidelines.
There's absolutely nothing wrong with "simple". In fact, in many cases, simple is good!
Your focus should be on making things easy, intuitive, and conform to user's expectations. Modeling your app after other applications, particularly those included with Windows, is a good way to make sure that you're on the right path.
Breaking new ground is dangerous territory; leave that to the big shops like Microsoft. Even new UI concepts that have panels of user interface/experience experts behind them like the Ribbon are slow to be accepted and even slower to be adopted. This isn't where a small software shop wants to be. There's no advantage in being out on the leading edge here.
The recommendations you'll inevitably get to use third-party controls are well-meaning, but ultimately misplaced. There's nothing wrong with the built-in controls and a lot of reasons to prefer them.
Once you've got a functional UI that conforms to the standard guidelines for your platform, the next step is real-world testing with actual human users who have never worked on any aspect of your application.
If they like it and they find it easy to use, then you know you're on the right path. I've never heard a user complain that something looks "too simple". Google Chrome is a huge hit precisely because it's simple. Remember that you're not designing a web page here. Desktop applications are very different.
In fact, it blows my mind how often questions like this one get asked here. Why do desktop app programmers spend so much time wishing that their app looked "cooler"? These are often the same developers who complained loudly because Visual Studio 2010 switched to WPF, broke a lot of their standard expectations, and got a lot slower, with little noticeable benefit. Sure, it looked cooler (unless you didn't like the color blue), but there was little in the way of functionality that the UI change alone was responsible for. Look at the apps you use every day. Do you really pine for a more whiz-bang UI? Or do you appreciate them and integrate them into your workflow because of how they work—because they conform to your hard-learned expectations of how a standard Windows application should work?
Make your app work like that.