Need It, Want It, Gotta Have It
There are places where you can bend in a build's design, and there are places where you can't. Without the things listed on this page, your build simply will not function; at best it will be extremely frustrating to play and at worst it may simply make no progress at all.
A good guideline is to aim to have about 400 Life per Act of the story, concluding with about 4,000 Life as you complete the main campaign and enter the endgame mapping system. By level 80 - our suggested level for planning for a new player - you'll want to have around 5,000 Life, give or take. Ranged builds can get away with slightly less, melee builds want slightly more.
Gear alone is not enough to get this much Life. You'll need many Increased % Life nodes on the Passive Tree, and one of the main tricks to good build design is to figure out how to get the offensive passives you want while still branching out to the Life nodes scattered around the tree. 150% Increased Life from the Passive Tree should be considered a dead minimum, closer to 200% is safer. This % Increased Life acts as a multiplier to the Life on your gear, so you'll still need plenty of Life from your equipment as well.
If you're using alternative forms of health, like Mana (with the Mind Over Matter keystone) or Energy Shield, you'll need slightly higher totals. For a Mind Over Matter build I'd recommend aiming for at least 4,000 Life and 2,000 Mana in the endgame; for an Energy Shield build I'd shoot for 6,000-7,000 Energy Shield with reasonable gear.
Life comes mostly from your Passive Tree, but Resistances usually come more from your gear. They're the main reason you can't simply use all offensive Unique items: most Uniques don't come with much in the way of Resistances (at least not by endgame standards), so you'll need to sprinkle in some Rare items for extra resistances.
Capping resistances at 75%, accounting for the 60% penalty you'll get as you go through the story, is essential. You'll need at minimum 135% Fire, Cold, and Lightning resist to hit this cap (since 135 - 60 = 75). Chaos resistance is less important for most builds, but still nice.
If you find yourself low on resistances, you can get more by using more Rare items with high Resistances on them or by using item bases like Sapphire, Ruby, or Topaz rings that come with large resistance implicit modifiers.
Sometimes you have a great idea for a build that doesn't match the section of the tree you're in. Maybe you want to play a Blade Vortex (a gem with a high Dexterity requirement) build with a Marauder or Templar (who start far from the Dexterity sections of the tree). It is absolutely possible to do this, but it requires you to plan ahead.
One easy way to get around an inconvenient requirement is to use the large +30 attribute nodes on the Passive tree. These nodes give +30 to an attribute other than the one whose section of the tree they're in. For example, you can find two +30 Strength and +30% Intelligence passives in the Dexterity section of the tree. Other options include amulets with large attribute bonuses: Lapis, Jade, and Amber Amulets come with Intelligence, Dexterity, and Strength implicits respectively, and they can also roll large bonuses (on the order of +50 to an attribute) as modifiers. Relying on these too much can make swapping gear very difficult, though.
Without a reasonable amount of damage, you won't be able to stop yourself from getting swarmed. Even if you can avoid outright death, clear speed is important to farm up items without having to spend forever doing so. Durability is all well and good, but at the end of the day, you've got to kill things.
As an example minimum for a level 80 proof-of-concept, I'd recommend aiming for at least 100,000 DPS in Path of Building with some reasonable configuration settings for a single-target situation. It's hard to give a universal standard here, though, because different builds function so differently and PoB doesn't account for everything. For example, the Rain of Arrows Guided Build included with this guide would only display as doing something in the low thousands, but Rain of Arrows hits many times and the build also benefits from an Impale effect Path of Building does not (as of this writing) model.