Mark said:
Some people play the game as purists, some tweek and house rule everything, some fall somewhere in between: Who are YOU?
Before my game starts, before chargen, i go thru and change rules. Most of these are either simplifications (i always try and bring in new players so simpler works well) OR they are changes specific to my upcoming story and setting.
These focus on things players will need to know before chargen.
After i get characters, i usually make a few tweaks here and there. These are made after i see the characters, see their "stories" and are designed to help my campaign (which is at that point being planned with PCs as stars in mind) focus on their particulars.
The way i see it, a published game is a mass market thing, a compromise designed to appeal to as many as possible, but made without:
1. specific knowledge of me and my preferences.
2. Specific knowledge of my campaign's story, its star characters, etc.
3. Specific knowledge of my players' preferences.
Armed with those three bits of knowledge, i can take the "store bought" system and make it a better fit to our tastes.
heck, believe it or not, i often add spices to my "store bought" foods as well.
same basic principle.
Sometimes its a few rules (a little salt and pepper) but sometimes its a majotr overhaul (cook it myself) like eplacing hit points with damage save et al.