Considering the criteria were "impressive" and ".308," lets work with the former. Too many math challenged public schoolers have abused the latter with the M1.
Using the flathead analogy, sure, it's impressive for it's time. However, it's cyclic rate, durability, and foot pounds of force output were made inferior by better designs. What criteria would anyone put on evaluating "impressive" if it's not based on obvious gains in power or versatility - something that can be measured and expressed in numbers?
Compared to the new Coyote 5.0 DOHC, the flathead is anemic and inefficient. Since ".308" pretty much limits the power output to an established level - say, NATO milspec ammo, what's left is the weapon. The AR10 is lighter, has less moving parts, is easier to clean, and more accurate. It can be made more accurate more cheaply, and doesn't need complicated presses and gunsmith level workmanship to get that accuracy.
Now, in terms of "impressive," what rates better for the M14 crowd? Name it, and I already mentioned a superior feature in the AR10. How can a weapon that is a collection of inferior designs be more "impressive" than one that is considered the pinnacle of .308 progress?
While it's certainly arguable that the Sengalese AR10 - it was issued, after all - doesn't have much better sights, the modern issued AR10's used by American and British forces at least allow mounting an optic over the receiver on a solid rail, not a cobbled together tin cover or side mount. And, being a universal pattern, isn't restricted to a proprietary clamp like the HK91. Darn optic mount cost more than the Aimpoint I had on it.
Nobody is even trying to compare feature for feature, just spluttering on about their romance with a curio and relic. If you can't quantify what's "impressive," then at least rationalize it. If an old wood and rusty steel gun that most soldiers hated for being heavy and kicking a lot is the epitome of what symbolizes firearms excellence, spell it out.
One line grunts chanting a mantra leave things a bit vague.
How about, "I love the feel of white walnut stained to simulate select grain better woods, the less than crisp and plain workmanlike inletting around the grainy parkerized finish of mass produced heavy iron parts that better designs eliminated..."
I've got an October '64 Winchester 94, sure, I appreciate an old gun for what it was, but I'm not about to consider what was run of the mill as being the most impressive. That's like saying the '65 Galaxie 500 with 289 is the most impressive car that year. Well, when your perspective is a bit broader - that's the year the 289 Cobra's won the FIA championship.
Same size motorvator, much better, lighter chassis than the Galaxie. Since ".308" is a standard reference for the discussion, what's left is the weapon. And to be "impressive," it should be a winner. The AR10 is still being issued, NEW, for use in combat, isn't that more impressive than guns not even available unless refitted from used?
Doesn't "impressive" also include design features that others include in their offerings? That SCAR in .308 is mosty based on the principal features introduced by the AR10. How impressive is it when most of the competition copies what you do?
Divorce the emo from the empirical, and you're left with the AR10. If "impressive" is all about sensory input and romance, I can't disagree. Old guns have that, largely because we didn't have to depend on them and put up with their quirks.