Presumably the number of speakers is a factor, as a language cannot change if nobody speaks it (is this even true in absolute?)1, but it does not necessarily follow that more speakers results in greater innovation. —Mandarin Chinese and English have roughly the same number of speakers globally but do they feature similar levels of innovation? If not then which factors are more important here, is it the number of native speakers, the geographical spread of language speakers, political fiat, or some other consideration?
Another factor, which jumps out as being a potentially major one, is existing dictionary size. One would think that once all the things are named and all the actions defined a language's innovation must naturally retard, but here too there are other things to consider.2 Perhaps this is not the case in languages used by cultures that show great innovation in technology or manufacturing, many brand names and former brand names have become fully-fledged words. Perhaps it is not the case at all, maybe a greater quantity of words in a language actually increases the rate at which new words are adopted.
To what degree linguistic purism a factor? It seems as though some purist efforts have limited effect or perhaps even increase innovation within a language such as the attempts of the Académie Française that often results in a sort of doubling effect where both the new native word and the loan word it seeks to replace enter into usage to varying degrees. Others, such as Arabic or Icelandic, seem to have greater resistance to outside influences on language. In cases like these however, do the efforts to resist actually result in greater conservatism compared to languages that make little attempt to do so or is this primarily due to other factors such as perhaps geography for Icelandic or religious/political reasons for Arabic?
In short, are there any universal factors which affect the rate at which languages change, what are they, and which of them are most influential?
1: A dead language such as Old Egyptian surely remains conservative for the time it is out of use. However, if people try to rediscover it at a later date this presumably changes; an Egyptologist attempting to interpret the language and getting things wrong could be seen as the Old Egyptian language innovating if this new interpretation becomes accepted and relatively widespread.
2: Of course one cannot name all the things or actions, but there does seem to be certain universal soft limits, e.g. most languages only name between two and eleven basic colours before resorting to other methods for describing new ones.