I was amazed by the number and quality of Google+ responses to the Swords and Wizardry Appreciation Day. It is pretty amazing the actual depth that this segment of the OSR actually has.
One thing that really stood out is how easy it is to houserule S&W. It is not just that they have the rules available in editable text format but that the rules themselves are very robust to tinkering. This was one feature of more modern RPGs that I found difficult -- if you changed the assumptions of the game designers then the system showed problems.
The classic example is magic items in a D&D 3.X/Pathfinder world. It is well known that the system presumes a certain level of magical gear for the players. It also assumes that it is easy to make magical items. Without these features, characters may well not operate as expected (the Monk 1/Druid X is much more formidable, for example).
But with S&W, the alteration of a single rule rarely has repercussions across the system. Sure, you can break it if you try to. But the game plays well with many tweaks -- some of which are actually discussed in the rulebook (ideas like giving +1 to hit to two weapon fighters, or no memorized wizard spells above level 6 but rituals instead). This allows for a lot of freedom for groups to tinker and try to find the optimal balance.