What are the qualities of a good weld?

Welds may be specified by fairly detailed codes which will set out the exact properties required for a specific application. Some general criteria are :

Penetration: how far the weld penetrates into the joint, often expressed as a percentage. This is a combination of how the joint is prepared and set up, the process used and the current/voltage applied. It is possible to have both too little (weak joint) and too much (poor finish in the back side of the joint) penetration.

Defects: including porosity caused by contamination of the weld due to inadequate preparation/cleaning or inadequate shielding, craters caused by reducing current too rapidly at the end of a run, cold starts (poor penetration at the start of a run)
Undercutting: thinning of the parent metal at the edges of the weld usually caused by not enough filler metal relative to current.

appearance: welds should have a reasonably smooth, uniform and consistent appearance
Oxidation: welds need adequate shielding from the atmosphere either by inert (or partially inert) gas or flux. Some metals esp stainless steel and titanium may also require shielding on the back side of the weld. Excess oxidation can contaminate subsequent passes, weaken the joint or require refinishing and can harm corrosion resistance.
Profile: depending on the application welds may be required to be flat, concave or convex. For fillet welds, a fillet radius may be specified.

Distortion and heat effects: shrinkage of the filler and uneven heat input can cause distortion of the finished assembly. This can be mitigated by careful control of heat input, planning order of welds on complex assemblies and proper clamping, joint preparation, and setup. Some metals including stainless steel and high alloy steels are particularly susceptible. also, high alloy steels and cast iron may require pre and post heating to mitigate residual stresses