I think the only ethical issue present in the gaming world is when real life money is exchanged by the user for something to get ahead of another person opposed to something that is acquired through the playing the game normally.
I see no issue in collecting 100 gold coins to get an extra life in Mario, it is a sense of accomplishment even if it is not required. It is harmless, it is a small ploy that might help shift games or maintain interest/create fans which developers/publishers need to do.
A more recent example would be for the call of duty: Modern warfare franchise. I think it is perfectly normal and acceptable to unlock new weaponry, be it purely aesthetic or something that is far superior dependent on achieving something, i.e. attaining level 30 or getting x' many heads shot or w/e. It is a computer game after all, you should be rewarded for working at the game.
However, I deeply oppose paying for content to get ahead of the next player as it is deeply exploitive and companies know this. This is where it becomes unethical; exploiting the knowledge that players want to get one up on their competition by getting them to part with their money.
Again, using CODMW but on a lesser scale, I think it is deplorable to charge an additional £10-20 quid when purchasing the game to get a code to unlock a weapon in advance. The reward is to have something that other people don't have and it is a really small example that shows how F2P models have their place and could be commercially viable.
On a more sinister scale would be any one of the many MMO's or Muds out there that allow you to play for free but require the user to pay for additional content be it weapons or armor etc. Perhaps the average user won't expend vast quantities of their money but enough do to make this payment model successful and worth doing- DnDO stated that they are finding people spending more money on the F2P model than the usual 6-15 pounds monthly subscription charges. This model exploits those who wish to better others or have the new shiny and will endlessly pump money into the system to yield better weapons, crops, orbs or whatever is on offer.
I'm really against this as I've experienced it for many years, essentially since the first fully graphical MMO's appeared, examples being Final fantasy online and world of warcraft. Prior to their arrival, the MUD scene (text based versions of MMORPG'S) was incredibly popular. Several games I played were frequented by excess of 100+ players (this was a lot specially back in the AOL days of paying for the internet per the minute). Upon the arrival of MMO's, many players left MUD's behind and alot of MUD's died due to a lack of population. A lot of MUDS's could no longer continue to develop the game and monthly charges were often waived in favour of F2P; users often had to pay money to receive buffs or new equipment.
The problem is how quickly it consumes people. It is like smack, not everyone does it but enough do it that is warrants a whole illegal drug trade to appear.
It is a horrible system which not only consumes people and their wallets but also ruins the games atmosphere and community. F2P Muds games suddenly become less about achieving something in the game as a group, i.e. slaying a certain difficult monster as a group or having a amazing role-play to what I could only describe as a, my dick is bigger than your dick competition. It got to the point in certain games where you were literally forced into buying new items/weapons to progress or to have a chance to compete with another user and in doing so one could easily spend more than the previous monthly subscription.
F2P has developed so much as it is now mainstream and it will only further worsen the gaming addiction. F2P is essentially the latest Heroin.