Breaking Changes in XNA 4.0

According to Shawn Hargreaves these are the Breaking Changes made in XNA 4.0.

A longer version here

• Replaced StorageContainer.TitleLocation with a new OpenStream API
• Replaced individual graphics renderstates with a new state objects API
• Premultiplied alpha is now enabled by default
• Made it easier to use SpriteBatch with custom renderstates and custom shaders
• No more SpriteBatch vertex shader magic, so you can easily position SpriteBatch text in a 3D world using BasicEffect
• Vertex buffers are now strongly typed, each one having an associated VertexDeclaration
• You no longer need to specify a VertexDeclaration before drawing, because this is implicit in your choice of vertex buffer
• Vertex buffer sizes are now specified as number of vertices rather than bytes
• Replaced GraphicsDevice.Vertices with a new SetVertexBuffer API
• Each rendertarget now has an associated depth buffer: no more DepthStencilBuffer type
• RenderTarget2D now inherits Texture2D: no more GetTexture method
• SetRenderTarget atomically sets all rendertargets, rather than changing just one layer
• Replaced Effect.Begin and End with a new EffectPass.Apply method
• Removed the low level shader APIs (VertexShader, PixelShader, Set*ShaderConstant)
• Removed EffectPool, StateBlock, GammaRamp, ClipPlane
• Removed triangle fans
• Removed point sprites
• Changed the Color type from BGRA to RGBA byte ordering