I think one of the main reasons for CoD4's success is due to the ingenious perk system. It has added miles and miles to CoD4's "replayability". The game is incredibly well balanced despite the countless combinations possible via perks and classes, which also points to the fact that the perks never took over the game. A skilled player with nothing but the default perks and weapons available from the start will still own any lesser skilled player who's reached level 55.
I've found after reaching level 55 and unlocking almost all achievement awards three times over, that I tend to play more often with weapons readily available at the early levels and the perks have more of a psychological influense on my style (and confidence) of play than actual numbers being tilted in my favor. The better player still rules, regardless of perks in most cases. I frequently get owned by guys using classes you'd think wouldn't do too well, but 9/10 I will dominate other level 55 players who load up with the flashiest weapons and perks. For this reason, I applaud IW for a very well balanced game with the only real anomaly being hackers and the occasional glitcher that will ruin (unbalance) the game for everyone else.
If you ask me, I think it's a very fair and balanced system that also retains the replay value of the game for a very long time!
I enjoyed BF2 when it first came out too, but it wasn't balanced at all and unless you levelled up quickly as soon as the game was released, it became increasingly hard for new players to stand a chance. This isn't the case with CoD, hence my opinion re: perks. The expansion to CoD1 added Jeeps and tanks in the WWII setting and was actually quite enjoyable, but also created a one-sided fight as soon as one team got the upper hand.
Oranges and oranges... At the end of the day, the better players usually prevail and the rest start whining!
