So what makes a rules set a commercial success?
These are rules which dominate their period/genre:
Without concrete data I'm only guessing, but there are some obvious names:
Warhammer Fantasy, Flames of War, Warhammer 40K, Warmahordes, DBx, Field of Glory.
Bolt Action is a rising star. I'm tempted to add Infinity as it always seems to feature heavily in any online stores.
Bolt Action, aka Warhammer 1939 aka Flames of War Skirmish. Generic, bland, unimaginatively copies tried-and-tested Flames of War/40K mechanics. Bound to be successful.
Out of Game ExperienceI don't know about you, but I spend far more time collecting, painting and assembling armies than I do playing. I'll break this down into a few subsections.
Are the miniatures interesting and flavourful? Is there more than one faction? (Not many people stop with just one army). Are the miniatures easily available?
Collecting & Army Building
A "points system" and/or "army list" is invaluable for collecting as it allows you to build acceptable armies for pick-up games or tournaments. I'm not going to go into the "scenario vs points" argument here but there is simply no reason NOT to include a points system. If you don't like 'em, ignore em.
Scenarios rely on organization and planning - which may vary between clubs, and from week to week. Games like Tomorrow's War who refused to do a points system for "philosophical reasons" simply hurt the long-term viability of the game - and ironically can frustrate scenario-builders who want a rough "rule of thumb" to balance scenarios. In fact, with scenario-only books like Tomorrow's War, there is little point in getting more than one rulebook for the entire gaming group.
In Game ExperienceEasy to find opponents
I have two Warmachine armies (even though I dislike the game on principle) as I know locally I can always get opponents (if I am that desparate). Again, points systems and army lists are important as it allows me to turn up and quickly get playing. These games are popular for tournaments which allow you to get in some solid gaming.
No fancy equipment or terrain requirements
Nearly every game listed above uses the humble d6 and does not use any elaborate markers or templates. This includes things like terrain - the prime barrier to Infinity is the terrain requirements - not everyone wants to spend 3 weekends making "enough" terrain to make the game playable.
Simple, consistent mechanics, little record keeping
Not too many charts, math or modifiers, and no written orders thanks! The game needs to be simple to learn. However some complexity is required or the game will come across as bland or shallow. Rules are usually concrete (i.e. true line of sight, WYSIWYG). Familiar mechanics are a bonus. Many of the ex-GW writers continue to re-make the same game - heck Empire of the Dead is LOTR with d10s.
Sigh. This is a personal bugbear, as games usually go overboard in this area (Malifaux, Warmachine, Infinity to name a few) which can result in players winning though having a better memory of the 157 sub-rules. Nonetheless, "Special Rules" are important to add flavour between factions and makes your troop units unique.
One of the reasons IGOUGO is popular is it allows a high level of player control, allowing you to control your forces relatively unhindered during your turn. All the more popular systems have very simple command and control - usually "stay within 4" of the other models in the unit" - and very basic morale systems (units test when take 50% casualties, and either pull back or flee the field).
Games should fit in the 1-3 hour range. This allows you to play multiple opponents, so you do not just have to play Fred with the terrible body odor the whole evening, and prevents games from "dragging" too much.
Chain of Command. Interesting, inventive, a little chaotic. Will be popular with groups or individuals, but will never achieve mass market appeal. Basically, is the subtitled French comedy against the latest Adam Sandler movie.
A case study:
Bolt Action vs Chain of CommandBoth have many interesting factions and readily available miniatures - you can use any WW2 manufacturer. However Bolt Action is closely supported by Warlord's own miniatures line.
Though Chain of Command has suggested platoons and strives for historical accuracy, Bolt Action has both a points system and an army builder and seems geared for competitive or tournament play.
Bolt Action is gaining momentum so it will be easy to get opponents familiar with the rules - or to convert them over from 40K/FoW, given the familiar mechanics. Chain of Command is a different "style" of game which may not instantly appeal - and has the usual chaotic mix of mechanics typical to a Lardies game. Bolt Action is going with the established audience. There's a commercial reason so many movies are sequels.
Both games have a sensible amount of special rules, but Chain of Command has a much more complex activation system which is a mini-game in itself. The focus on "friction" means units may or may not be able to activate. In Bolt Action, you are guaranteed of moving each and every unit. Bolt Action, with its familiar mechanics and streamlined feel, plays faster.
Is Bolt Action a better game? I'd argue that Chain of Command offers far superior gameplay. But it would take a braver man than I to bet against the superior commercial success of Bolt Action.