Do you go for a single stat improvement or more flexable ablity. Perhaps its will,fortituge and reflex saves. Maybe loads of special powers, spells or abilitys. Could it be loads of skills. Raw power or finesse. A good mix of character classes.
I go for lots of versatility, good strengths and no apparent weakness.
Sign of a good build: Survives long high hp, ac and abilities to make sure all enemies die before he does
Bad build: A strength so obvious people will become immune to hit, and the build then getting unviable.
High sneak attack low defense, good example
Mage with high death spells low everything else is another bad build.