Following a question on the core rule set on the ModSecuirty mailing list, I would like to list some of the key properties of the core rule set. The focus of the core rule set is to be a "rule set" rather than a set of rules and the properties below are all derived from that:
Performance - The core rule set is optimized for performance. The amount and content of the rules used predominantly determines the performance impact of ModSecurity, so the performance optimization of the rule set is very important.
Quality - While there will always be false positives, and the core rule set is young, we spend a lot of time trying to make the core rule set better. Some of the things we do are:
Generic Detection - The core rule set is tuned to detect generic attacks and does not include specific rules for known vulnerabilities. Due to this feature the core rule set has better performance, is more "plug and play" and requires less updates. If you want to patch known vulnerabilities you may look for rules from gotroot or convert snort rules such as those at bleeding threats, but you must select only the rules that apply to you, otherwise performance may suffer.
Event Information - Each rule in the core rule set has a unique ID and a textual message. In the future we are going to add classification using a new tag action in ModSecurity, as well as longer information regarding each rule using comments in the files themselves.
Plug and Play - We try to make the rule set as plug and play as possible. Since its performance is good and it employs generic detection, and since the number of false positives is getting lower all the time, the core rule set can be installed as is with little twisting and tweaking.
To get deeper into the core rule set you may want to read the presentation I gave about it in a recent OWASP chapter meeting.