Few things to note when deciding a build for a hero(In order of importance)...
1) HP. You have to make sure your hero have sufficient HP throughout different stages of the game. For me, I normally make sure my Hp is equal to my level, EG: Lvl 12 = 1.2k Hp(At least, if you are the tanker or carry, make sure you have more to avoid dying).
2) Map mobility. Is your hero able to travel across the map fast enough? Heroes like AM, QoP have blink which enable them to travel across the map easier while heroes like Furion or Pit Lord have teleportation skill. Heroes with high movement speed like sladar can move around faster too. Heroes with BoT or Tp will enable them to get more FF opportunities or ganking possibility, which results in more gold. With better mobility, the better his farming ability gets... That is Y all heroes should get boots, more speed, lesser time spend on travelling... In addition, it helps in chasing heroes which enables you to have more kills. Items like blink dagger are good to chase heroes.
3) Escaping device. All heroes should at least have 1 or more escaping device, so that he or she will not die so easily. Heroes like PoTM or BH have escaping skill. Heroes like Lion or Rhasta have disable, which enable them to run away. Make sure your hero can run away from heroes when dying or it will be very diffcult to farm up big items like Eaglehorn or Mystic staff. Dying constantly will also reduce the hero's farming time and ganking opportunities, which both result in $...
4) Dmg Per Second. How much dmg can this hero release per second. Like AoE dmg, targeted dmg or Dmg by normal attacking. If a hero is unable to release huge Dmg within a short time, it will not be useful during gank fight, which will result in lesser gold due to the inability to kill. Buying items which help the skill snychronise better and release more Dmg would be good. This will normally be your CORE itmes. EG: Butterfly, SnY, Dagon.
6) Counter items. This items cannot be decided before the game start. It depends on the opponent line-up. Get gem to counter Riki, blademail to counter ursa, hex to counter am and stuff like that. This items normal come after the above few...
Hope this above information helps=)
Nites...
