100+ hours first playthrough, haven't killed moder yet, but farmed plains already. I'm very addicted to the game, and plan to do several playthroughs.
The most annoying part of the game is the upgrade cost to benefits. As you all know, the level 3-4 upgrades to weapons, and especially heavy armor cost insane amounts of ingots, and the benefits are almost non existent.
What I'd do to change this are to either decrease the cost, or maybe increase not just the armor rating, but the benefits or skills that the sets grant. As things are, there is no incentive to upgrade stuff last lvl 2, it's a waste of time.
Another thing that bothered me from as early as bronze age, is that there is no visual change from upgrading gear. I'd expect a lvl 2 weapon to have some visual indication, decoration, enhancement, to distinguish it from lvl 1, and so on. If I spent all that time grinding for upgrades, at least let me show it off!
Source: https://www.reddit.com/r/valheim/comments/10uccbn/the_biggest_thing_id_change_in_game_is/