[RPG] What are the advantages and disadvantages of allowing a player to play a large creature

Combat in the old-school systems is immensely faster. Yes, there are also fewer interdependencies between actions than in WotC-era editions, which reduces the amount of tactical wrangling over what each player should do; but even apart from a simpler choice of actions, everyone taking actions at once just vastly simplifies the process of getting the round done.

Having players all take their turns together simplifies the planning of the round. Because order of the players' actions when moving or attacking isn't a rule-bound thing, it's easy and comfortable to declare an action, realise it's a bad idea, and quickly roll that back to do something else. Getting everyone's actions to work together cleanly is just a much simpler process due to the flexibility of moving all at once. Even in a tactically-complicated round where people are thinking hard on the best way to execute things, even one where people are declaring actions and then rolling them back because they suddenly see a better alternative, the rounds are quick and there are few mental, logistical, or rules barriers to moving the action along at a brisk pace.

Since the players are all going pretty much together, they don't tend to "check out" and stop paying attention to the round. The gain in speed from not having to remind the next player that it's their turn, and not having to repeat everything that just happened to get them up-to-speed, is not inconsiderable.

Keeping things in order is actually simpler because there is less to the mental model the players have to individually retain and collectively synchronise. You just don't have to keep track of turn order or think about who should do what when on which turn to make a tactic work. There are no wait actions, no triggers, no finicky positioning work to make sure people are in the right place when someone casts a certain spell three initiative-turns later in the round.

As a long time gamer, I've noticed that I loved creating my character's past and personality, but typically abandoned what I created anyway as I got to know my character better through play. I wanted to believe in DAS, but ultimately leaned toward DIP.

In the last few years, I've become interested in narrative writing and have learned why this is. In storytelling a character's past, education, appearance, intelligence, mannerisms, etc. (the kind of stuff that exists on a PC's character sheet) is considered characterization not character. True character on the other hand, can only be determined by the choices a character makes particularly when the choices are risky or bear a price. This kind of thing tends to happen in game-play not character creation.