Is the CR system predicated on this approach?
The CR system was apparently designed with no feats, no multiclassing, and standard array for PCs. I think that the CR system is at best a very rough guide, and has already been mentioned, most helpful to newer DMs and players. I think that once the DM is comfortable and is familiar with his players and their characters, the best thing to do is jettison the CR system entirely and to start designing encounters based on judgment.
I wondered if others thought the game would be too easy with:
1. Multiclassing alone
I don't think that multi-classing alone would make too great a difference. You may get a player who dips into a class for a level or two to gain some key abilities, but I don't think there are too many such combos that woudl be so overpowering. I honestly think multi-classing is generally not all that effective, especially given how sub-classes and backgrounds can really help achieve very similar effects.
Alone, I don't think that feats are that overpowered. Yes, each one is potentially potent. Most are pretty robust in what they provide. The combat oriented feats certainly seem to be the popular choices. Feats tend to narrow a character's area of specialty so that they excel in one area with perhaps a small sacrifice in versatility. However, when balanced against an Ability Score Increase, which is a smaller boost that is applied more broadly, I don't think feats are as bad as many claim.
I think this is one of the areas where people will see huge swings in power. It's amazing that when people roll stats, they never seem to roll poorly. When we used to roll stats in the earlier editions, we were all very soft on each other and would allow plenty of rerolls and so on.
The only way that I see this working is if the DM sits and watches the rolls, and does not allow any changes. So if the player rolls 3 crappy scores, too bad. The problem then is when the other player rolls 4 strong stats and 2 medium ones.....then you have one player whose character will be very strong, and another who may be very weak.
So far, of all the options, I think this one has the most potential for messing up the game balance.
4. A combination of any two of these
Combining any of them with the Rolled Stats is going to be a big deal.
Other than that, though, the other combos won't be as severe. More so than the individual options alone....but there are combos of multi-classing and feats that have a synergistic effect. Not so much for me to restrict feats or multi-classing in any way in my game.
I think one of the big things I've seen in online discussions that can really skew balance is if your entire group of players has a powergame mindset, and how closely they all work together to build their party. I've seen examples where every single player choice for their character was made with the party's performance in mind. So all races were chosen based on having darkvision, and so on. I don't mind my players making sure that all the essential "roles" are covered, but once they're all choosing feats and races and spells and class options in unison, I think things have gotten out of hand.
Others would say I'm crazy, but I think that's the thing to look out for. When every decision is made with combat application in mind, all done with the expectation of being part of a party.
What I am looking for is perhaps a bit more struggle. So far I have had fun but and looking ahead to both designing and playing in campaigns which get the heart rate up a bit. Not insta death but real challenge. Would reducing options help? Or is the answer always that the DM should make things harder on their end?
I don't think you have to reduce options, other than having either point buy stats or standard array. Feats and multi-classing are fine, but just keep an eye out for abuse. Most of the time you'll likely be okay. I will say that as a DM, you can and should increase the difficulty of your monsters from time to time, by adding feats or abilities or increasing damage or whatever you feel is appropriate.
The worst thing a DM can do is to see an issue of some sort...the party is walking through all encounters....and not do anything about it. The DM does have to put in effort, and should expect to do so a bit more if his players are having to easy or too hard of a time.