That's a pretty broad question.
For analog layout, the main concern is noise; you want to minimize any external noise from getting into the analog circuitry. This means paying attention to component placement, routing of analog and digital signals, shielding, ground paths, etc.
For I/O, the main concern is speed (and minimizing switching noise that might get into your analog circuitry). There are also sometimes issues of impedance matching, differential-pairs, reflections, etc. Depending on your requirements, you may need to pay attention to keeping trace lengths the same to minimize 'skew'.
If you have specific questions, ask them.