First, I am not complaining, because I am doing poorly - in fact, with 5 players I made the top 10 in the record amount of games, but I am not enjoying the game nearly as much as with 4 players. The problem is that the second age, which is supposed to be the heart of the game is way too loose:
1) Resources are way too abundant in the second age. Yes, with 3 players resources are way too tight, but in 4 players there are some truly intriguing decisions about whether to, say, win a military conflict or be shut down from the loom or water. Furthermore, there are really no strategic hides, because both brown AND gray resources are abundant, so it is almost impossible to shut down any of them - in 4 players, you can make powerful resource blocks, especially as Olympia or Alexandria.
2) No tough decisions about the military. Usually the most tactical decision in the second age is how much military to play - you want to play enough to win, but not too much to sacrifice the resources, points, and stages. In 5 players, you do not have to think about it - if you will overplay on the military, you will be compensated with Ludus. In that sense, the game is very similar to 3 players, where the second age military is the key. However, in 3 players you face a clear resource trade-off - do you play military or take necessary resources. In 5 players the trade-off is minimal, as you just take the military and due to the abundance of the resources you collect them later.
I think Arena should always be 4 players - it is clearly has the most variety of tactical decisions making the game the least mechanical. In 5 players it is really difficult to finish last (I personally seldom finish last even dead drunk): you decide in the beginning what is your strategy and then you mechanically implement it - yes, you are not guaranteed to win, as the card order always matters, but you will essentially never finish last if your plan is even half-sensible, because due to the availability of resources you will always be able to implement it - and remember that avoiding last places, not winning, is really what drives you to the top of Arena. With 4 players - the second age is really the centerpiece of the game with all the tactical decisions, with 5 players the interest is really gone after you made your plan at the beginning of the game - the rest is just a routine.
1) Resources are way too abundant in the second age. Yes, with 3 players resources are way too tight, but in 4 players there are some truly intriguing decisions about whether to, say, win a military conflict or be shut down from the loom or water. Furthermore, there are really no strategic hides, because both brown AND gray resources are abundant, so it is almost impossible to shut down any of them - in 4 players, you can make powerful resource blocks, especially as Olympia or Alexandria.
2) No tough decisions about the military. Usually the most tactical decision in the second age is how much military to play - you want to play enough to win, but not too much to sacrifice the resources, points, and stages. In 5 players, you do not have to think about it - if you will overplay on the military, you will be compensated with Ludus. In that sense, the game is very similar to 3 players, where the second age military is the key. However, in 3 players you face a clear resource trade-off - do you play military or take necessary resources. In 5 players the trade-off is minimal, as you just take the military and due to the abundance of the resources you collect them later.
I think Arena should always be 4 players - it is clearly has the most variety of tactical decisions making the game the least mechanical. In 5 players it is really difficult to finish last (I personally seldom finish last even dead drunk): you decide in the beginning what is your strategy and then you mechanically implement it - yes, you are not guaranteed to win, as the card order always matters, but you will essentially never finish last if your plan is even half-sensible, because due to the availability of resources you will always be able to implement it - and remember that avoiding last places, not winning, is really what drives you to the top of Arena. With 4 players - the second age is really the centerpiece of the game with all the tactical decisions, with 5 players the interest is really gone after you made your plan at the beginning of the game - the rest is just a routine.