So in terms of sameyness -
Having just a fighter and just a Rogue for martial classes wouldnt be a problem if they were actually adaptable classes
What do you want the 5e fighter or rogue to do that he cannot?
or if feats were able to adapt better (sentinel seems like a great defender feat except really we only get one reaction and it conflicts with the protection fighting style),
I'll solve that one for you - be a cavalier
they didnt seem to want you to be able to specialize or be functionally more than damage damage all damage.
A battlemaster with good Charisma and rallying cry and inspiring leader and healer does a lot more than be a damage (leader)
A fighter that grapples can stop an enemy in their tracks (control)
A fighter that trip attacks is slowing an enemy and giving allies bonuses to hit it control)
A fighter can cast utility spells to buff himself or possibly others (leader)
This is what 4e classes were accused of because of roles but roles actually were leveraged to create more approaches to the battle right away for martial types yeh for non-casters.
There were 2 primary parts of a 4e role
Role Mechanic - The defender ones were the only role mechanics I can think of that had a unique feel between classes in the same role.
Powers that supplemented the role - This was your 2[W] + small side effect ability that 90% of all powers were modeled after. You had to really try to not take a samey feeling 2[W] + small side effect power with every character in the group. Heck even most dailies went to 3[W] + moderate side effect or 2[W] + bigger side effect. There were a few gems that if you optimized you could find that would eliminate some of the sameyness - but overall the game was very samey. For example,
I'll use encounter power to do:
Player 1: 2[W] + slide 1
Player 2: 2[W] + grant +1 attack for next ally
Player 3: 2[W] + make enemies next attack be at -1
Player 4: 2[W] + pull 1 (and mark it since I'm a fighter)
Also, an added layer of sameyness was the encouragement to refluff everything.
Most of 5e's choices they give to a player are not samey. Subclass choice is huge.
Classes all have a different mechanic - despite nearly all of them using extra attack.
Barbarian = Rage
Fighter = Action Surge + Number of attacks
Paladin = Divine Smite
Ranger = Hunter's Mark
Monk = Flurry of blows
Full casters are alot more samey in mechanics - but there's still a huge difference in playing a bard, a cleric or a wizard because of the types of spells each have access to.