Beyond the other issues outlined, a 4e game with more than 5 PCs starts to get extremely unwieldy. I consider 5 PCs to be the upper limit; unless the 6th is a striker...and able to resolve his/her actions quickly. This is for multiple reasons and each interacting with the prior issue to compound the problems:
1) Time/round goes up.
2) The potential for time/actor goes up (more players equals higher probability that one of them is math deficient or paralysis through analysis driven).
3) Encounter budget goes up which in almost all situations (except for an anomalous minion extravaganza) means considerably more total HPs to ablate...especially with multiple Brutes and/or Soldiers.
All told, if you do not have (i) a considerable number of strikers making up those 6-10 slots, (ii) considerable action economy buffing leader/striker synergy (warlords granting barbarians MBAs), and (iii) a highly functional group with advanced tactical acumen and quick turn resolution then you will be starring down the gun barrel of a grueling experience. I often wonder if these nightmare scenarios (6-10 PCs, tons of enemy HPs to ablate, horrible synergy and multiple, pure defenders and leaders, poor tactical acumen, long individual turn resolution) are what causes people to say that 4e combat is a "grind". Our combats are fierce and quick; 30 seconds/PC turn (including immediate actions, if any), perhaps 1 minute for my own as GM, 4-5 rounds on average (6 - 8 on L + 5 boss fights). With 3 PCs that is 10 -12 minutes per combat. Hardly a "grind".
The bottom line is, in the above scenario, it is highly likely that average rounds of combat would jump from 4-5 rounds to possibly as many as 10. There will be a much larger number of enemies for the Defender to be controlling than during a standard combat with 4-5 PCs (which may be 2-3 typically). If a Defender is controlling 5-6 enemies for most of those 10 rounds and getting hit twice a round for his surge value (and being healed for roughly the same value and expending a surge/heal)...a low level Defender will run out of surges by round 6 or so.
Point being, there is a point where the game breaks. My guess is that @
KaiiLurker may have found it.