Has made reasonable all-round contributions to article writing (~1,000 mainspace edits) as well as areas such as tagging for speedy deletion, discussing articles for deletion, reporting vandals and other admin related activity
or
Has shown specialised dedication in one of the above areas (even in the absence of balanced experience in several)
If I am in any doubt one way or the other, the kinds of things that are likely to make me swing one way or the other are:
Support
Oppose
Candidate stays "cool under fire" during the RfA
Candidate comments on all opposes, becomes defensive
Acknowledges evidence of occasional incivility or poor judgement
Justifies behaviour, shows no sign of learning
Gives at least one example of admin activity to be undertaken
Cites non-admin activity as the kind of thing they will undertake
Clear evidence of improvement since previous RfA
No evidence of (or no time for) improvement
Interacts helpfully with others via talk pages
Little (or unhelpful) interaction with others
Writes well (grammar, spelling, punctuation and argument)
Hasty or careless writing
I should say that I am much more likely to express an opinion at RfA if I see an approval rating between 60%-80%. I reckon that there's little point in piling on either support or oppose votes if the decision is clear cut. So my lack of contributions on some RfAs is likely due to that; having said all of which, I do sometimes contribute where the current percentage is outside that window, as time and motivation allow!