(All of the stuff below this is of course my personal opinion)
I have played, Legends of the old West, LotR, Warhamster, WH hysterical, Bolt Action, Flames of War, IHMN, Flying lead, Drums and Shakos, 45. adv, DBA, the list goes on and on...
Everything is advertised as realistic, ultra-fast, no book-keeping, great mechanics ...and I always believe that!

but always I found something was not quite right. So here comes the question(s)...
I have to run another convention game and would need to design rules for that again, so therefore I am curious what you guys think, what needs to be in, what is unnecessary?
1. How important is it to have different individual shooting/fighting skill levels. Most of the games assign a skill level to the figures but some try a different approach, e.g FoW uses the opponents skill level to hit, LotR only uses just a opposed D6-role no matter who fights... So what should be used to determine the outcome of a fight/shot? Is it the skill of the commander, the weather....? What is it?
2. Rules need to be simple and self contained. Very often rules are advertised as easy but the actual complexity is uploaded into individual talents that are then allocated to the figures. So the complexity is just disguised and it requires annoying referencing as no one can remember all the individual special rules. So what has to be in the stat line to make it interesting but not too complex?
3. The action is on the table not in a book. I don't like rules that require constant table referencing. Shoot a soldier with a pistol who wears a helmet... look up pistol range table and cross reference with the armor table and troop skill table...the roll on the tabletop should immediately tell me if it was a success or not (opposed roles seem to be the solution here?)
4. Cards on the table look bad between nicely built terrain...(playing cards to determine the initiative, counters to show the current mood of the trooper etc) The Bolt Action solution seems ok (small and informative) but some just turn figures round or place on the side to show effects...
5. The difference between big and skirmish level games seems to be an issue, or is it? Would it be possible to have something that covers both. Some system try to do this by classifying figures as "heroes and troopers", and use summary skills for the groups.
6. The warhammer approach is

, I have to sit and wait for half an hour until my opponent has moved all his troops ... A system needs an action/reaction mechanism. Move shoot, next move and shoot also seems to be strange too, why not have a more flexible approach, actions that can be done in any order and will trigger an opponents response...? I wanna shoot first and ask questions later (well maybe shoot and then move, you know what I mean)
I have designed my own simple rules for conventions/demo games but still look for something out of the box.
So anybody any idea what system would do all of this? Or maybe comes close...
And if not what are the most important aspects
oh I forgot...no separate rules for tanks...so if a pistol is shot a a tank it should not do anything without referencing to special rules (as point 2. above, no special rules uploaded into the weapons stat line...)