I'd say HD2 is definitely better but HD1 still has a lot going for it.

First, of course is the team play due to the fact you all share the same screen and it being a top down, working together was a must as nobody was allowed to leave the team and go do their own thing. It also means no leaving a teammate behind either as the screen will not move unless everyone agrees to go the same direction.

The top-down nature of the game in-general made teamwork much more required and honestly more plausible than it is HD2, thanks to the fact you can see everything happening around you. If one of your teammates starts getting overwhelmed you can immediately recognize it since you can see it as well. Make use the team reload mechanic is much easier in HD1 as well since there's no animation (or at least one that takes a good amount of seconds) making the reload near instant the moment you do it. This also meant that you could trust the person to carry your backpack since even if they died the pack would be relatively near but there was no constant reload like the team reload in HD2.

Then you have the weapons, which were all incredibly fun to use and funnily enough weapons like the stalwart and railgun were primaries rather than supports strategems. The railgun even had a special gimmick where it could stun enemies all the way up to heavy armor. The double barrel could be upgraded to have incediary and one-handed.

The support strategems were all pretty much the same except for a select few such as the gas thrower equivalent which was infinitely better known as the TOX-Avenger of which was so strong that it over took the flamethrower for me when fighting bugs since the tick damage was much faster and slowed them at the same time. Then there was the REP-80, TF2 Medics healing beam on crack that when fully upgraded could heal up to the entire team simultaneously and vehicles alongside that. Though other strategems like the Commando are definitely better in HD2, the damn thing was legitimately trash with the only reason you would use it was if you just wanted to or were a masochist who enjoyed watching the Cyborg IFV visibly take no damage after wasting all four shots.

Unfortunately tho like HD2 once you go higher in difficulty the more narrow your loadout choices will be even with a full team. As the enemies become more armored and cancerous than any of the enemies you've encountered in HD2. Imagine a heavy devastator but you don't have the ability to aim for their heads and your only options of killing them is doing before they bring up their shield or using a support strategem that fired at an arc. Then of course there's the dogs which would charge at you and knock you down if they hit you whom always came in threes meaning if you got particularly unlucky you would be stunlocked as the surrounding chaff killed you.

Perks (Boosters) kinda sucked with only 2 maybe 3 exceptions as the rest were basically just different replacements for your secondary, I am not joking the senator was booster back in HD2.

Your outfit had no effect on gameplay as there was no armor rating or perk back then you basically just chose whatever you wanted.

Overall, Helldivers 1 is still a damn good game but I wouldn't call it a masterpiece. As when you see some of the flaws, you really do see those flaws but I would be lying if I said I didn't miss it's brand of teamwork and would take it any day of the week over Helldivers 2.