Certainly in the gaming groups that I play in, one of the most complained about thing by other Chaos Players is the Champions of Chaos special rule. This is a rule that is given to all characters, and it has 2 effects. Firstly, you must challenge and you must always accept challenges. Secondly, if you beat down an enemy character a combat, you roll on the Chaos Boons table, which can give you many 'rewards'. But why is it so hated?
To answer this, I'm going to bring up 2 examples from my past, one from each of my Chaos Lords. The first was my Axe of Blind Fury Chaos Lord. I was playing a Welsh ETC player at the Toy Soldier tournament and while one of my Land Raiders hung back because charging forwards would just get my guys charged by Dreadnoughts and tied up, the other bombed forward and dropped 2 squads into the enemy lines. The Axe of Blind Fury Lord went into a squad of Purifiers, was forced to Challenge the Justicar, poured 11 attacks into him and killed him 8 times over.
Now, this was problematic for 3 reasons. Firstly, I had easily caused enough wounds to kill the entire squad of 5 Purifiers, so why would you just kill the Champion when you can kill everybody? Secondly, as a reward for his epic kill of the single Purifier, Khorne decided it would be absolutely hilarious to turn him into a Chaos Spawn. Thirdly, this Land Raider and the squad inside were essentially trying to push the wave to stop the Grey Knights getting to the objective in the left hand lane. Now, instead of having a nasty Khorne Lord and 8 Marines chomping through the enemy lines, I had 8 Marines and a Chaos Spawn desperately trying to hold the Grey Knights back.
The second example comes from yesterday, when I played against an Ork player, where I charged a mob of 20 boyz with my Black Mace Lord, killed the Nob, and then immediately killed the Nob, but turned into a Daemon Prince before the mace could complete it's swing forcing toughness checks. Eh? Firstly, surely it takes longer to turn a Chaos Lord into a Daemon Prince (see Dawn of War) than it does to complete the swing of a mace. But secondly, the only time you would really take a Chaos Lord is either for a Brand Bike (why not Heldrakes?) or for horrific beatstickery. Especially with the latter category, the Daemon Prince you are given cannot hold a candle to that guy. Now I understand that you have to take into account Aspiring Champions, but the simple way to fix this is to make (with the exception of Terminator Armour) all wargear transferable, so long as the Wargear that the Lord took can also be taken on a Daemon Prince should it be purchased as a HQ choice.
Now here's another point. I charge a squad of 8 Chaos Space Marines with dual close combat weapons into a Daemon Prince. I normally get 33 attacks on the charge. But because of this rule the all charge in, the standard marines are then forced to pause and have a little dance while the Aspiring Champion goes in and gets munched, but even if he does get to strike, I have lost a total of 28 attacks in the first round of combat, and then when we get to the next one (if we get to the next one), I only get 14 attacks. So where I would normally be getting 52 attacks over 2 player turns, I am instead getting 19. I don't understand how this is fluffy?
If you're going to include a rule to show the background of the army that's fine, but what makes a Chaos Space Marine more likely to challenge than a regular Space Marine? In the Chaos Codex, it mentions a Chaos Warlord named Roghrax (also my League of Legends name), he is mentioned because he is a Khorne Lord who took more skulls than he knew what to do with. There is also a Chaos Warlord named Geoff mentioned, he is in there because he beat down a Purifier Sergeant once before the rest of the Purifiers beat him to death. Have a guess which one of the two I just made up.
I guess this is a bit of venting, in my mind Chaos Space Marines are brutal killers, not Assassins with equally expensive meatshields who are obsessed with just killing the higher ranking blokes. Chaos doesn't care who you are or what you've done, Khorne cares not from whence the blood flows.