In Path of Exile, the trait for every piece of item is designed very rigorously. If there is big difference in the level items, the value for the lower level item would never exceed the value of higher levels. Then, for the items of close levels, how can we identify its value and choose the most proper ones for us?
Firstly, you should know what is Affix. Affix is the attribute that belongs to your gears. For example, +17 to dexterity, adds 4-7 fire damage. The Affix can be divided into Prefix and Suffix. According to the introduction from the official site, For example, a Long Bow with the "Frosted" prefix modification and the "of Impact" modification would be named "Frosted Long Bow of Impact". Rare items have their names randomly generated from a list of rare names and do not follow this principle. Magic items can only have a prefix and suffix, so if they are already named X Item of Y, you can't use Orbs of Augmentation on them."
Then we need to know how the Affix is formed. For Magic Items, there is at least one Affix, and there are two Affixes at most, and these two Affixes is combined by a prefix and a Suffix. There will never be a blue item with two prefixes or two suffixes. For the rare items, there are at least four Affixes (Regal is out of the option, as Regal is the only way to make yellow gears of three Affixes), and six Affixes at most. Among these six Affixes, there must be three prefixes and three Suffixes. It means you will never find a piece of gear that provide four kinds of resistance as resistance is belong to the Suffixes. Regal Bow used to be really welcome in game as it is of high value. If you can use the right Orb on it, you can make a really awesome weapon.
To avoid the items of lower having Affixes that are better than the item of high levels', the game developer added many restrictions to the design at the very beginning. In the other words, you can not own a gear of incredible attributes just by the picking the loots, you have to optimize it by the combinations. For example, you can only find the increase move speed on the shoes, but not on the helms. You can find Affixes to increase the defense, but no Affixes about attack on the shoes. The item in game that has the widest range of choice for the Affixes is the ornament. I like the normal ring, especially the gold ring which can help increase the MF. I can use the Orbs to change it into rare ring. The weapon that has the widest choice for the Affixes is the staff. On a staff, there could be all physical/elemental Affixes and all the Affixes about the magic. If you want to know what kind of Affixes can show on certain pieces of items, you can search the details in WIKI by putting in "Unique Items". You can conclude something from the classified introductions. I would also like to make the conclusion for you in the following updates. The other thing you need to know is the range of the values. We can also get the answers from the WIKI.
After knowing the Affixes of the items, and the restrictions to the item levels and positions, we know what the limit of the item is. If you have a 40-level Bow, you have three elemental prefixes then. It really worth to spend some Exalted Orbs to improve the potential of the item. As for which item worth you to spend the Exalted Orbs to improve, we will come to that point later.