A bit late to the bonfire party down by the train wreck, but I hope I'm in time for seconds
The problem isn't with just the game, it's with the gamers.
If the entire group as well as the DM is cool with some party members pulling more weight than others, there's no need to adjust encounters or house-rule stuff.
You can throw the entire brigade of Orcs at those level 4 characters, since you know that 2 Greenbound summons and 3 entangle later they'll be manageable, even if the Druid is pulling more weight than the Warlock.
If the entire group want to feel like they're pulling equal weight, there's 1 of 3 things that can be done:
1. Everyone plays classes that can pull equal weight.
2. The DM starts making encounters that the weaker classes can start contributing more (see the earlier comment on monks, mobility and rockslides).
3. The more powerful classes hold back.
Guess what?
Nothing of the above is the wrong approach.
The problem arise when:
1. Some people want to play certain classes or concepts that aren't mechanically viable/inferior by normal standards (let's say normal standards = Tier 3).
They get angry, envious, sulky or some other kind of negative attitude when they realize they're not the big guns. They can't handle that and whine that the other classes are "powergamers" or "muchkins" or perhaps just sits quietly not doing any roleplaying at all, sulking.
What do I know, there's plenty of different ways to express this.
Less politically correct members would probably call this "small penis syndrome" but you won't be hearing that from me
I had this situation once where I....
[spoiler]DM'ed a players with a lvl 1 Ninja. I told him Ninjas weren't good and that you could do the concept with another class, but he said he didn't care. Now, given his rather stupid tactical decisions, this Ninja was rather suboptimal even by Ninja standards. Applying poison to a shuriken _in combat_ with no training in doing so (that's a full-round action doing nothing and risking poisoning himself). Running straight down a glass-covered road after being seen by guards armed with crossbows (and I asked him explicitly if he tried to dodge in behind a building. He answered "no, I run straight down the road.
He managed the balance check to not fall, but the two crossbow bolts that landed him at -7 HP he couldn't handle.) and other stunts like these two brilliant ideas...
[/spoiler]
2. People playing weaker builds realize just how much the DM adjust in their favor, feeling like they can't do things properly without help.
This is objectively true, but the reaction to this situation can both be positive and negative.
3. The stronger builds get shit so that the weaker classes can feel better.
This is what seems to be the most common past occurrence for many people in this topic, which might explain their attitude towards the topic at hand.
I can personally take having a bit harder stuff thrown at me if I play something more powerful than my friends. It happened once when we were forced to partake in gladiatorial combat. My opponent had better stats than the other two, since I played a cleric and the rest of the party was a Bard and a Evoker Wizard. I didn't mind.
HOWEVER.
There's a lot of bad DM's, which thinks "shitting on good builds = promoting bad ones" which isn't true.
Due to vastly different experiences with this sort of thing, some people associate suggestions like "challenge the wizard more (implied: so that he has to work harder for it and not just solo everything)" as "let's give a lame-ass excuse to shit on the caster". Which, given their less-than-competent DMs, isn't so suprising.
Challenging Casters = Good.
Shitting on Casters = Bad.
Challenging Casters in a way that kills non-casters= Bad.
Challenging Casters in a way that doesn't mean dead non-casters = Very Difficult to Near Impossible.
4. The problem with more powerful builds holding back is that they might feel like their options are being limited. While this is objectively true, it is a matter of perception whether or not they feel like this is a bad thing. Attitude.
Now, to set some things straight:
Casters > Non-casters. No one in this entire 7 page discussion have claimed that non-casters can equal casters in power.
More encounters does not mean more strain on the casters than on the non-casters, unless said non-casters have infinite supply of resources (HP, smites, rages). Then one might argue that you give infinite resources to all but 1 player, which is unfair. The chance that he appreciate this "anti-favoritism" is abysmal.
More difficult encounters means more strain on the casters, but unless crafted VERY WELL, will prove a lot more straining for the non-casters.
Casters have solutions to every encounter. HOWEVER, here comes something I like to call the Boyscout Fallacy (their motto is "Always prepared", so I find the name suitable).
The Boyscout Fallacy states that even though casters probably have a spell as a solution for any situation, they might not have that spell available at the moment it's needed. Barring COP TO-cheese, there's a possibility that a certain encounter can not be handled optimally with 1 spell, since there is no 100% certainty that the caster have prepared said spell. The other version, for spontaneous ones, is that casters can probably squeeze out any of their known spells that can handle a situation, IF the situation is one where it can be optimally handled by a spell known. This might not always be the case.
That said, there are a lot of different encounters that can be solved near-optimally with a selection of very useful and versatile spells, or spells that just are very powerful. This goes for both prepared and spontaneous casters.
Preparation, planning and scouting (magic or otherwise) all help to pick out the most optimal spell selection for the ones that prepare spells.
But it's not always a 100% guaranteed fact that the casters can handle the situation optimally with the spells they currently have access to.
Handle optimally does not necessarily equal just 1 round of casting, but it might.
Whether or not a player group or the wizard player himself find it acceptable for their DM to change some things in the upcoming battle just so that the wizard can not trivialize it by himself, is totally up to the persons involved. You cannot say that this is "Good DM'ing" any more you can say it's "Rampart Douchbaggery". That depends 100% entirely upon the gaming group themselves and their attitudes towards this. There is no objective truth here, it's only a matter of perception.
"Oh, so he spread them out and mixed the opposition so I can't handle it with 1 spell. Interesting, I like a challenge, let's try this tactic then / God damn, he's mixing stuff up just to mess with me, fuck, this isn't fair. I can't handle this".
Same situation, different attitudes. I could go on, but I don't think I need to.
Now for a personal opinion:
Casters trivializing encounters with 1 spell is something that happens a lot. I find this at times annoying, but have learned to cope. It does not mean that EVERY SINGLE ENCOUNTER ALWAYS CAN BE HANDLED WITH 1 SPELL. Just a lot of times. The limit for "one too many times" vary, some react after the first time it happens, some won't react ever. Something to keep in mind when discussing stuff like this.
Long post is long, but I really couldn't shorten it down without risking to be misunderstood.
Will probably happen anyway, but I'm a min-maxer, I try to Maximize my viewpoint and Minimize misunderstandings.
