To answer this question properly I should write a whole chapter, but I keep it short:
There are three major differences between a WPF application and a Windows Forms application: Layout, Render, Presentation
Layout:
WPF layout system provides a greater flexibility in arranging the elements on the fly. It is based on the Element Bounding Box (as opposed to precise pixels in WinForms) and Measure and Arrange mechanics (as opposed to UpdateLayout in WinForms) that automatically and dynamically finds the place for each element without any need for a manual update.
Basically, all elements bounding box are measured first and then are arranged using multiple methods such as Measure, MeasureCore, ArrangeCore, MeasureOverride, etc.
Unlike WinForms, where you have a pixel-perfect size for everything, in WPF you have much more options and complexity such as Width, ActualWidth and DesiredSize (and even Transforms as LayoutTransform) for the same element.
This is why
As you type in a WPF TextBox, its width might increase and push other elements away or even push some elements into a new row (like the menu bar you've observed)
As the size of a control changes, it affects the available space for other elements. So their size and location might change accordingly.
When the window is being re-sized or resolution is changed, it immediately updates the layout and changes the size of elements in order to fill or fit the space. Here you'll find out more about Layouts.
using Margin alone (without using layout capabilities) to arrange elements is not the best idea in WPF. As it's the WinForms mindset which isn't much helpful while developing WPF.
Render:
WPF uses double data type for Layout (as opposed to pixel-perfect WinForms) and therefore you might see the edges blurry sometimes, but it can be avoided with SnapToDevicePixels=true.
WPF is much more efficient in utilizing the GPU to render a GUI. Try a grid of 30x30 TextBoxes in a Windows Forms application and a WPF application. No matter how messy you write the WPF, it never blinks and it still runs much faster than Windows Forms. Even adding a handful of animations, visual effects and styles on them does not hurt your performance like in Windows Forms.
Remark: To avoid a speed decrease and blinking in a Windows Forms application, you should set DoubleBuffer of the form to "true".
You can use any Transform as RenderTransform to easily implement smooth zoom/rotate, or develop custom GPU-based shader effects, and much more in WPF. (I think everyone agrees that doing such things in WinForms is feasible but real pain and you most likely will give up and move to GDI+ or DX if not out of frustration then because of the bad performance.)
And the last and the most important:
Focus on presentation:
When develping WPF Applications you have to stop thinking in Windows Forms: No more UI events, accessing controls by their names and writing logic in code-behind and start to think in WPF: Binding, Commands, Resources, Styles, Templates, Converters, DependencyProperties and their callbacks.
The real power of WPF lies in separation of 'View' and 'Logic', Which can be achieved using the MVVM pattern.
It makes the most visually-complicated problems quite simple and easy to develop and easy to write Unit Tests for.
Once you got the hang of it, you will realize there's no limit in how you can present the data or show off an awesome GUI looks.
If you've planned to switch to WPF, you've made the right decision. Always stick to MVVM and AVOID CODE-BEHIND AT ALL COSTS! (i.e. unless you are doing a pure UI operation: do not write code in .xaml.cs files, do not access x:Name in cs files and avoid UI events.)
