Not quite true. In bell curve distribution, a +1 is more valuable if your chances of success are near 50%, and less valuable if your chances are either very high or very low.
Exactly; my point was that diminishing returns are more thoroughly showcased in such a system. You can clearly see how the value of the +1 diminishes as the game progresses. Let's say I'm playing a roll under system which uses 3d6, and I set 10 as the 50% mark via my choice of game mechanics. A person who is untrained at a skill and has a low default of skill 5 has less than 10% chance of rolling what they need; skill 6 is better, but still less than 10%. Likewise, on the high end of the skill spectrum, a skill 15 character has somewhere around a 95% chance of success; adding another +1 to that character (for a 16) bumps him up to around 98%... not a huge difference.
With a flat roll, I have an equal chance of rolling every number on the die each time I roll. Granted, the range of what I can roll and still succeed or fail is going to change depending upon the challenge at hand. D&D levels tend to assume I get a certain amount better over time, and it's that part which I have some concern about. Even considering the more flat math, how much will the full game assume I take a certain amount of +1 bonuses over time? How far can I get from that assumed power level and still function? This leads to other considerations as well:
As Pem said (and rightly so,) 4th Edition made an attempt to keep to-hit right around 50% regardless of level. If Next were to keep that model, +1s would remain important throughout the game. If Next does not do that and PCs simply get better as they level (with their to-hit threshold becoming wider,) it makes +1s less relevant at higher levels, but it brings up other problems. One of those problems is that D&D has static defenses; as such, if everyone is running around with virtually no chance of missing, we end up with combat turning into a contest to see who can take the most HPs* the quickest. Assuming that is the case and also assuming monsters do not keep pace (by virtue of being built differently than PCs and running on a different system,) then we run into a problem similar to what early 4th Edition had in which the PCs were so much better at combat than their adversaries that it prompted a math rehash via Monster Manual 3 to fix the game. Assuming that is the case, but assuming monsters do keep pace instead, then I feel that there is a risk of running into some of the problems that high level 3rd Edition had in which combat became something akin to rocket tag -if you could win initiative and go first, you could attack and kill the enemy before they had a chance to do anything because their chance to defend against what you're doing is so low.
I'm sure there are plenty of ways to avoid that. As I said before, I'm in no way claiming to be an expert. I'm simply discussing what my perception and thoughts currently are. It may be that I'm completely misguided in my thinking. Until I see parts of the game which aren't available for me to see yet, it's hard to say.
*Edit: That was worded somewhat poorly. Combat usually is won by who can take the most HP the quickest. What I meant was that -in a game where everyone was assumed to hit and everyone also has static defenses; meaning they do get hit... I'll be honest. I know what I want to say, but I'm unsure how to word it. Think of it this way: Imagine you were watching a soccer game in which neither team was allowed to play defense and simply took turns scoring until time ran out.