I think Call of Cthulhu 5.6 will go down as one of the most concise and efficacious renditions of the game, noting the new 7e rules additions and I recommend it.
The major issue some groups have, rules wise, is:
- the disconnect between the Characteristics and the Skills which both have different scales,
- the use of the Resistance table and, following on, the lack of explicit contested rolls for skills (the Resistance table provided is nominally just for Characteristics, not skills).
- Some people have also cited that an investigation can grind to a halt if you lean too heavily on rules-as-written and roll dice to determine the success of investigating clues. There is also a supposed ‘whiff factor’ in combat, where continual failed rolls can be frustrating.
Personally, I think a lot of these issues are overstated, but I have never particularly stressed about rules-as-written and don’t tend to worry about fudging a few things in order to develop the plot.
It is worth having players keep a written or mental check of 1/5 scores for all percentiles, all Characteristics x5%, and note that the basic formula for a contested skill rolls can be worked out as: 50%+ skill minus opposing skill (not hard, really). It is also worth rewarding good descriptions of investigating activity with automatic successes (rather than rolling dice all the time). If combat scenes are important, note that players wanting to play competent fighters ought to have skills in the region of 70%.
The default assumption of the game design (and what marks it as distinct from most others) is that the PCs are likely to lose - either being killed or going insane. This is the horror genre, after all, so what would be considered heroic actions in other games are pretty stupid. The enjoyment in the game usually comes in the thought provoking plots and premises of the scenarios, or in the thrill of solving a mystery. Read through each scenario carefully so that you know it well enough to adapt to the players actions, rather than making them feel railroaded. If they get stuck, make liberal use of the Luck, Idea and Knowledge rolls to spur things along.
