Quasqueton,
I've obviously hit a nerve here but in all honesty, I'm surprised at your flabbergastation. Allow me to elucidate my point of view while expanding upon your points:
In the first instance regarding a Golem, why give the players something that only one of their PC's can affect? A forward thinking DM would perhaps round out the encounter with a few other beasties already there as well. Either that, or not including such an unbalanced encounter in the first place. Simple planning.
In regards to the Devil, the archer and the Wall of Fire. Commonsense is going to tell you that the Devil is not going to play the pin cushion hiding behind the flames if it can help it. It's either going to burst through it and starting ripping throats or kindly vanish to a different plane if it feels over-awed (Not likely
). By the way, the wizard who cast the Wall of Fire after realising his mistake would in all likelyhood stop concentrating on the Wall and start focusing his or her efforts on a more fruitful endeavour.
As to suggestions about being a very good DM I think I can see where most of your difficulties lay and I'll hopefully give you some excellent advice.
Firstly, Dungeons and Dragons is NOT the DM vs the Players. The Players have monsters and encounters to worry about without trying to take on the DM as well. DM's who take such a perspective tend to piss their players off more often than not. The DM's job is to facilitate a good time for all - including themselves. The key to this is to know your rules, know the PC's, their Abilities and Equipment, know your NPC's, Campaign World and Monsters and I suppose most importantly know your Players.
The DM should know the rules like the back of their hand. If you want to keep one step in front of the Players, it is essential. You need to be able to predict what can happen in a particular situation and when the unpredictable does happen, you have to be able to adjust to it very quickly. A full comprehension of the rules is imperative in such situations.
A DM should know exactly what different PC's can and can't do. Know what Feats and Special Abilities PC's have so you can give them encounters where they can use them. There is nothing more frustrating for a player than a DM who tries to neuter characters by circumventing their abilities. Let the Paladin smite the bejesus out of an obviously evil creature, give the rogue a bit of slack (Darkness or proximity) so they can use their sneak attack and let the Magic-users have fun with the handful of spells that they have. If you want to give the PC's a very difficult encounter, give them a way to escape so they can come back prepared for whatever monstrosity you insist on letting loose upon them. In essence, help them build up their characters rather than leaving them to say "God knows why I chose this feat, it's bloody useless".
A DM should be able to give color to their world through their NPC's and the world around them. Use accents, give visuals and most importantly provide variety - "not more undead?!". The issue here is to know your monsters very well. The use of random encounters is really just a crutch for unprepared DM's. A good DM will have planned what they want you to face and has no need for a random encounter table. Such planning means you don't have to tweak stupid encounters. When using a pre-written module, don't be afraid to tweak it here and there if you feel the changes are more appropiate for your PC's. Experience and Monster knowledge is obviously the key to this. For instances where they have to face a particular monster, you will find that it's normally attached to the story of the module. As such, you should have ways for them to find out information about their final enemy so they can prepare accordingly. At least you are giving them a fair chance.
And finally, know your Players. If you know that a group of players likes more in-depth role play, give it to them. If another group is a bunch of munchkin power gamers, give them the combat and XP they crave. When mixing the two, lean more towards combat if you can help it. Using the above should help make it enjoyable for all if not quite as meaty for some. I normally run two campaigns, one for a small group of role players and the other that includes everyone in a more combat related game. Keeps everyone including me happy. However, just as you can have poor DM's, you can have poor players. The trick is to either train them into better players (normally the best option), give in and give them encounter after encounter or more simply, don't invite them at all.
Hopefully, I have shown you that it's not difficult to be a good DM, it just takes time, practice and most importantly preparation. This is why most situations that seemingly require patience really just require a prepared DM.
Best Regards
Herremann the Wise