Once you have decided upon the Genre, and if need be the sub-genre, you need to figure out which rules set, if any you will use. This will probably require two sets of polls.
The toughest world design is probably "Rules Neutral". You design and write up the world without using any gaming rule mechanics. It is up to the end users of the world to translate the written description into the gaming mechanics they desire to use. This is probably most appropriate if the world will be used mostly for writing instead of gaming. The biggest issue I see with a rules neutral world design is magic for fantasy worlds, and high tech for science fiction. If you try and build a rules neutral fantasy world where magic requires hours of work, you will have a hard time using such a world with d20 D&D which has lots of spells that take only minutes to cast. In addition you have the problem of the effect of magic upon society. If you are trying to create a science fiction world with faster-than-light travel, your FTL drive needs to be compatable with at least one rules set. If the world is not for gaming use, the choice of rules becomes easier since you proably don't have to be as concerned with the numbers as for role playing.
The most time consuming choice is home brew rules. You have to not only design the world, but you have to spend time designing the rules that will be used to run any games set in the world. The process of designing gaming rules is beyond the current scope of this web site.
Genre specific rules are fairly easy, but as with any rules set, you are constrained by other peoples rules. There are so many different rules availiable that the second poll would have to be conducted based upon list feed back. Factors would be the actual rules, but also the cost for everybody to get a copy and easy of use.
The Universal Rules such as d20, GURPs, HERO have the advantage of being well known. Most of them have lots of genre expansions to make life easier.