I wish that the GM I am talking about was really horrible since, then, leaving the group would be a viable option. No, this falls in the category 'luxury problems' which are never done out of malice. But he isn't just a good GM when he's on a roll, he's usually splendid. A lot of thought goes into the adventures he writes, he prepares several hours before sessions, he loves what he is doing, he encourages creative choices. However…
1. He plays favourites
It's plain and simple. When he likes a character, the spotlight is solely on that one. It also happens when the players are particularly outgoing or vocal. The GM doesn't do this on purpose; he just gives this character an unholy amount of screentime and rewrites his campaign around the character. I understand being a cheerleader is sometimes necessary and I wholeheartedly endorse it; in fact, I think it's necessary to sometimes sit back and let character arcs play out. However, it is now usual that players (me among them) sit around for hours without having anything to contribute to the plot and being delegated to watching somebody else's story.
2. He plays favourites against his own judgement or comfort
In a current One Ring-Campaign, he's starting again to fall into this pattern. Two young characters clearly overestimated their chances and attacked a Troll (retreat is never an option for them), an encounter they wouldn't have survived if not for his intervention — he threw NPCs in the Troll's path, which the troll magically targeted. The adventure ended with the two of them being hailed as heroes. This is a pattern — he refuses to let characters die, rescues them with in-game methods and then lets them have all the in-game glory for misbehaving. There are never any consequences to killing NPCs willy-nilly, teleporting into the middle of an enemy army or insulting a noble in a setting that punishes lack of courtly manners with death. Quite the contrary. The GM is clearly frustrated by this behaviour in one particular player, but never does anything about it and devotes all the positive attention to him.
3. Oh, the rules-mongering
There is no point in getting acquainted with the rules, because he will rewrite them and set house-rules without getting the players' consent. In one five-year-campaign, he had three folders for just his house rules. Three! Not one of them was discussed with the players beforehand, and were just dumped on them. So much micromanaging.
4. When any of these problems are brought up privately and calmly, he dismisses them
Oftentimes, he even lashes out. When told about favouritism, he usually accuses the person of personal problems with whoever is currently favoured. When asked about his micromanaging and rules-lawyering, he will clamp down. He will just spam in-game reasons for some questionable decision until any complaint ceases.
As I said, it would all be so much easier if he was bad at his craft, but he is not. I will admit that I am really not the most diplomatic person around and I am probably still miffed that in one Fate campaign, I waited three years for the big reveal of a Fallen Angel on my characters shoulder, only to be treated at the end of a campaign as a two-minute-afterthought. (And spending one year in patient anticipation, while waiting around several hours in every session. You know, out of consideration. Hours and hours of preparation and private consulting with the GM, and it came to that.) This was two years ago, but it's my go-to example of what is wrong with this GM, and this pattern is coming back in style. When I brought it up, he complained about everybody and everything else (me being too patient, the other players not taking subtle hints, mysterious characters are a pain, etc.). But he never acknowledges fault. Other players have occasionally complained, only to be brushed off in a similar manner.
Short version: GM is great in general, but he refuses to let anybody die, plays favourites, rewards bad behaviour and dismisses any criticism to the point of lashing out. Any amount of diplomacy has failed. The only time he took it to heart was when a player passionately declared their frustration in an open WhatsApp discussion, after which things improved. That improvement is gone now.
Are there any in-game strategies to be recommended? Any ways to approach him, to reason or to express frustration, without outright leaving or sabotaging his carefully planned campaigns?
Best Answer
I am going to preface this with an observation: if you all have been gaming together for over five years, I assume that you are friends. You have a friendship, a relationship, with this person that goes beyond the table. If that assumption is false, and after five years of gaming together you aren't friends, then your frustration with GM "at table" style is sufficient to recommend the following: walk away0. This person will not change for you based on what some stranger on the internet (me) has to say2.
Otherwise, if your GM is your friend, read on ...
Four actionable items
Mostly fun. So far so good.
1. He plays favourites
You need to let the GM know that playing favorites is making the game less fun for the non-favorites. Do not sugar coat this, but be tactful. Arrange a non-verbal signal -- double wink, two twitches of the shoulders, something distinctive -- so that you can signal during play that you think it's happening again. GM will either act on that or won't, but at least you've offered a means to identify when "it's happening again."
2. Oh, the rules-mongering
This isn't over the line, unless the GM surprises you with house rules. If that's a habit, you need to spell out in clear terms, as above, how this damages fun. As long as the house rules are above board and known, a GM is well within the generic GM role to do this. It is important to present your distress with the in-character references: the characters would mostly know how the world works; it is only through the GM that you know what's new, and what's changed. To use a crude example: if gravity in our world only worked sometimes, the world would be a very a different place so we'd make different decisions.
3. When any of these problems are brought up privately and calmly, he dismisses them
Make a decision. This is on you. If on the balance, you have more fun than you don't, then suck it up and play on, have as much fun as you can. If on the balance this is a deal breaker, then you have to walk. Sometimes, the only thing that speaks to strong willed people is actions rather than words. (trust me here, I walk in those shoes, Type A personality) Make sure he knows (you all have been gaming together for a long time) that it is no longer fun for you. Then walk. See what happens. (And particularly pay attention to how the rest of the group reacts).
Are there any in-game strategies to be recommended?
The bulk of this is out of game interaction. Trying to solve this during the game creates a no-win situation for you, since you are not the GM, and if your response disrupts the flow of play, you may accrue negative responses from the other players.
4. Lastly: show by doing.
When you GM/DM for a session, make sure you do it right as you see it, and as you have described it. Between games explain why you do it differently.
GM's are human beings
GM's can be imperfect. Heck, most are. For the amount of money you are paying him, that's par for the course.
Are the shortcomings such that you are no longer having fun? Walk.
Can you live with them because there is still a lot of fun? Play.
1 To amplify on "walk away" slightly: it doesn't necessarily mean cut ties with the group, just declining to play that game. With board games, hanging out, or a game with a different group member as GM, no reason not to continue the relationship. (thanks to @fectin for the comment).
2 For a RL point of reference, the best advice I ever got before getting married was: don't assume she'll change. The same is true for friendships: don't presume someone will change for you. Offer advice? Yeah. Make suggestions? Yeah. Demand they change for you? Sometimes they will and sometimes they won't. They decide what they'll do, not you. That's how people are.