I was referring to spellcasting pre 4E.
How spellcasting in 5e is different from 3e & earlier (not really all that different - slightly fewer slots, even greater flexibility in using them, fewer restrictions on casting, etc) in't really relevant to how a Warlord from 4e would work relative to caster classes in 5e. How 5e casters differ from 4e casters (vastly more slots, much greater flexibility, greater acess to more powerful and versatile spell lists) is relevant.
Paladins I would rate as some of the most powerful classes int he game borderline OP
If that's how you'd rate 'em. I wouldn't. 'Borderline OP' doesn't mean a lot in 5e, balance is a lot looser, that border is wide and fuzzy, encompassing most of the game until the DM steps in.
and since most warlord advocates want the warlord as a support type PC plus healing plus some other abilities its to much.
I don't know what you think you mean by 'support type PC plus healing,' healing is at the core of support-capable classes in D&D. As far as 'plus some other abilities' no concept of the Warlord ever advanced has had any where near the range and depth of 'other abilities' on top of any support function than the extant 5e support classes have just by virtue of being casters.
The 5E rogue is the prime example of why at willa ction granting is a stupidly bad idea in 5E.
It's an example of how inconsistent 5e class design is, and how that makes adding a new class a little more challenging than in a more consistent game, but it's a minor consideration. Designers do have to think about how any new class or ability interacts with any/every other existing one, since those existing classes and abilities don't follow any consistent design parameters. It's not as 'modular' as it could have been.
In 4e, granting a basic attack as a limited at-will action-grant worked fine, because basic attacks were uniformly less powerful/versatile than attack powers. 4e design was just neat that way. In 5e is, perhaps ironically, more complicated.
Add 5E monster have less hit points than 4E
At lower level, sure, at higher levels, 5e monsters have more hps. The same with PCs, actually. 5e puts more scaling on hps/damage than prior editions, to make up for the limited sense of advancement from bounded accuracy.
There is an issue in that any action-granting ability would have to keep up with that rapid scaling, somehow. That could be addressed by having the scaling come from the Warlord's level, rather than the ally's - the ally just providing the attack roll and damage die, for instance. Not ideal, IMHO, I think the Warlord should be more about the ally than that, but it might be a workable alternative to granting full actions (including sneak attacks, extra attacks, spells, smites and who-knows-what-else) as a way of capturing the range of damage scaling mechanics 5e uses. Or maybe Mearls &co can come up with something better, they are the professional designers, afterall.