Summary
-Visibility of discourse to observer (particularly language of discourse and technology facilitated discourse)
-Influence of theological traditions on societies in which discourse is taking place
-Participant experiences (particularly with relation to religions in proximity)
-Reaction to discourse by relevant parties (participants, audience of discourse, local community, religious community etc - particularly their propensity towards violence)
Hmmn well there are quite a few different factors. Firstly it is important to note that there is a bias in terms of the Observer's awareness of when/where such discourse is taking place; If the observer speaks english primarily, then Christianity is one of more most prevalent religions in the english speaking world with atheism one which is either extremely well established or fastly growing (depending on which english speaking nation one looks at), the observer is aided by the capacity of technology to distribute such material among technologically developed nations (within which most of the more visible debate is occurring - enabled in large part due to shifting theological demographics and availability of technology) christian denominations are far more prevalent.
Then there are the domestic factors within the societies in which individuals with inclination to espouse dissent facilitated through technology (and less impeded by threats and incidence of violence) - these are typically religiously (predominately christian due to demographics within largely democratic nations) motivated interference in secular governance such as faith based initiatives to deprive people of appropriate education, health care, access to reforms etc. Participants also have personal factors, one is more likely to be familiar with religions with which one has access to - particularly in one's native tongue and with proximity. Of course, more personal factors would include previous experiences with local religious communities, even particular religious individuals, opinion on the policies (and implementation thereof) of religious institutions and more.
In terms of the reaction towards such discourse: Christianity is the largest religion in the world which has a contemporary lack of significant following among adherents of violence towards those who contradict them; in addition individuals are seldom motivated by Atheism (as opposed to antitheism for which there is more evidence) towards violence against those who contradict them, nor are there contemporary significant atheistic movements which have institutions which promote violence as a reaction to ideological differences. Therefore, both parties have a reasonable expectation that they can engage in dialogue without being subjected to violence which facilitates lively and visible discourse including the expression of disagreement.
In english speaking communities, the most common theological debates will be about religions that are prevalent in (or have a perceived impact on) the english speaking world. In mandarin speaking communities, the most common theological debates will be about religions that are prevalent in (or have a perceived impact on) the mandarin speaking world. But within those communities in which there is disagreement about ideological issues, the occurrence, visibility, character and content of such discourse is likely to be tempered by the attitude of the community towards such 'conflict' (and indeed subject specific differences may be present) and in particular by the attitudes of individuals and groups towards how to deal with the expression of ideological disagreements. In a community inclined to reasoned discussion, one can more easily air contention than one can in a community which sees ideological dissent as inappropriate and should be avoided or even worse as crimes and should be punished.
An interesting (and sometimes unfortunate) departure from this is that technology facilitates an inter-community spread of dialogue which increases the range of communication to include a wider audience within which there is naturally a greater chance to include those who will react violently - it is also sadly true that such individuals are not uniformly distributed (and cultural factors may facilitate or obstruct willingness to engage in reasoned discourse as opposed to violence). This is particularly the case when an audience member seeks to propagate the initial communication (possibly incorporating translation, editing and so forth) for the sake of engendering the perception of ideological grievances with a wider audience.