I really think that a Fighter with max Str18 or a Rogue with max Dex18 is still a very good PC (and it if it wasn't, it would mean that it is a MUST for every Ftr to reach the cap in Str, and I would hate to play a game like that!).
I can't comment on the rogue, but I think there game will generate considerable mechanical pressure to take a fighter's STR to 20.
Consider a 10th level fighter. A quick scan through the current bestiary reveals 10th level creatures with the following ACs: 12, 13, 14, 14, 15, 15, 16. That's an average of a bit less than 14 - we'll call it 14 for simplicity's sake.
A 10th level fighter has +4 to hit from class. Let's add a +1 weapon - then a longsword is +5 to hit for 1d8+1 damage before STR.
With STR 18, vs AC 14, that means the fighter hits on 5+ for 1d8+5: average DPA is .8*9.5 = 7.6 before crits. Of those hits, 3 in 16 are crits doing an additional 4.5 on average, so overall average DPA is 8.44375.
With STR 20, vs AC 14, the fighter hits on 4+ for 1d8+6: average DPA is .85*10.5 = 8.925 before crits. Of those hits, 3 in 17 are crits doing an additional 4.5 on average, so overall average DPA is an ugly fraction that, for convenience, I'm rounding up ever-so-slightly to 9.72.
That's a little over a 15% boost in DPA for pushing STR from 18 to 20. Allowing for the 0.8 hit rate, the 18 STR fighter gets just a touch more benefit from boosting STR to 20 than s/he would get from an item or feat granting +1.5 to damage. I think that would be a fairly attractive item.
The more important the crit is as a component of damage, the less that STR difference will matter (because the crit rate is STR-indpendent), and the bigger the damage dice the less the STR component matters. If I do my comparison with a greataxe fighter, for instance, it looks like this:
With STR 18, vs AC 14, that means the fighter hits on 5+ for 1d12+5: average DPA is .8*11.5 = 9.2 before crits. Of those hits, 3 in 16 are crits doing an additional 9 on average, so overall average DPA is 10.8875.
With STR 20, vs AC 14, the fighter hits on 4+ for 1d12+6: average DPA is .85*12.5 = 10.625 before crits. Of those hits, 3 in 17 are crits doing an additional 9 on average, so overall average DPA is an ugly fraction that, for convenience, I'm rounding down ever-so-slightly to 12.21.
But even here, the increase in STR is granting a little over a 12% increase in expected DPA.
Now comparing with advantage on the attack:
With STR 18, vs AC 14, that means the longsword fighter hits 24 in 25 times for 1d8+5. Of those hits, 111 in 384 are crits doing an additional 4.5 on average, so overall average DPA is 10.42 (very slight rounding down).
With STR 20, vs AC 14, the longsword fighter hits 391 in 400 times for 1d8+6. Of those hits, 111 in 391 are crits doing an additional 4.5 on average, so overall average DPA is 11.54 (very slight rounding down).
That's still better than a 10% increase in DPA. And for the greataxe fighters, it looks like this:
With STR 18, vs AC 14, that means the longsword fighter hits 24 in 25 times for 1d12+5. Of those hits, 111 in 384 are crits doing an additional 9 on average, so overall average DPA is 13.64 (very slight rounding down).
With STR 20, vs AC 14, the longsword fighter hits 391 in 400 times for 1d12+6. Of those hits, 111 in 391 are crits doing an additional 9 on average, so overall average DPA is 14.77 (very slight rounding down).
That's a bit over an 8% increase in DPA. At this point, I'll concede that there's not much at stake between the two scores. But even here, it's still granting better than +1 to DPA, which is in turn better than a +1 damage bonus (because, given some attacks miss, that's less than a +1 expected DPA).
Now DPA is not the be all and end-all; but for a fighter it's not nothing, either. Of course defence matters too: the longer you can stand up, the more rounds you have to deliver your DPA (though killing the monsters quicker helps your friends with weaker defences). And non-warrior fighters have other tricks to deploy besides damage. But unless those tricks (like the gladiator's Dirty Trick) give you a reliable way of getting advantage on attacks (and of course the gladiator doesn't get the increased crit range, which is very strong with advantage and is helping to dilute the importance of STR in the above examples) I still think you're going to want to max out STR.
Making STR add to damage only would be a more radical way of significantly closing the gap. For the longsword fighters without advantage, the DPA boost would be a bit less than 9%, and for the greataxe fighters with advantage it would be less than 7%. At that point, boosting STR would still be a viable option for improving your fighter, but I think other options would start to look more viable too.
EDIT: A very basic comparison to the rogue:
A 15% boost in effectiveness is going from needing a 15+ to succeed on a check, to needing a 14+. (Success rate goes from 6/20 to 7/20.)
An 8% boost in effectivness is going from needing a 9+ to succeed on a check, to needing an 8+. (Success rate goes from 12/20 to 13/20.)
I think if, as the player of a rogue, most of the checks I faced were 15+ for success, I would want that +1 pretty badly. Whereas the boost from needing 9+ to needing 8+ I could more easily live without.
4e maths is based around 9+ as the basic threshold for success. If D&Dnext is carrying that logic into its maths, than rogues may not be under the same pressure to max DEX as fighters are for STR, especially if they can get advantage on checks fairly easily.
It's the unrelenting nature of the hit point attrition game, and the fact that it adds to both attack and damage, that makes STR so important for fighters.