Here are some points I came up with myself:

Digital painting:

• Flexible for lots of visual styles
• Lots of resources available for learning
• Scales decently

Vector art:

• Easy to edit
• Scales very well
• Relatively easy to animate

Pixel art:

• Classic "game look"
• Relatively easy to work with at low resolutions
• Very very laborious at higher resolutions
• Does not scale at all. Better have that sprite size locked up by the time you draw it.

3D in general:

• More work than 2D
• After the model is created and rigged, making animations is easy in comparison. No redrawing.

Poly modeling:

• Specifically suited for angular geometric looking objects

Sculpting:

• Easier to create complex models than with poly modeling
• Specifically well suited for smooth organic looking things