andyrm +1
Enthusiasm important too.
Also, among the wide range of people I ride with there’s naturally a mix of abilities, but also I notice a difference between what I would call ‘good riders’ and ‘good cyclists’ – there’s plenty of guys in the Uni club who are super fast, with very impressive race CVs to back it up, but take them on a ‘proper’ ride and they’re useless – poor trail manners, lack of basic outdoorsy knowledge (map reading, ability to choose appropriate clothing etc.) and ill prepared – not taking tubes, tools, proper food etc.
Then there are others, not quite as quick (though still capable) who are much more complete mountain bikers, and the sort of people you want to be riding with on a big days in the mountains.