What makes a good rare item in Path of Exile 2
How to judge a rare item in PoE2: prefixes and suffixes, tiers, item level, open affixes and why 'good' depends on your build.
Prefixes and suffixes
A rare item can have up to three prefixes and three suffixes. Every modifier is one or the other, and an item can't have more than three of each. Life, energy shield and flat damage are typically prefixes; resistances and attributes are typically suffixes.
That split explains why some combinations are rare: an item with high life and high damage has used its prefixes on both, leaving no room for a third prefix.
Tiers and item level
Each modifier comes in tiers - higher tiers have bigger numbers - and the best tiers can only roll on items with a high enough item level. A low item level item can't have top-tier mods, however lucky the roll.
Within a tier the number varies too. When you search trade, a minimum value in the middle of the tier you want is usually much cheaper than the top of it, for very little difference in play.
Open affixes
An item with fewer than six modifiers has open slots. An 
Exalted Orb adds a random modifier to an open slot, and essences can add a chosen type - a gamble or a cost, but often a cheap way to turn a decent item into a great one, if the mod you want is the right type for the open slot.
Open affixes are worth checking before you buy: a ring with two good suffixes and an open prefix may be one lucky exalt away from being better than a far pricier finished one.
Good for whom?
There's no such thing as a good rare in general. A ring with cold resistance, life and cast speed is excellent for a caster who needs cold resistance and useless for an attack build that's already capped.
Judge an item by what it adds to your character after what it replaces is gone. That's why the same item can be a big upgrade for one player and a downgrade for another - and why tools that simulate your whole character beat looking at item text.
Things that make an item worth less than it looks
- Corrupted items can't be changed further, so their open affixes stay open
- Mods your build doesn't use, however high they roll
- Requirements you can't meet - an item you can't wear yet is no upgrade
- A base type that's wrong for your defences (evasion on an energy shield build)