Sorry for the late reply, I've been away on a meeting.
1. Moltovir already posted a good link

Open wounds is a useful mod in Hell, where monsters regenerate life quite fast, and it's very nice against DC (Diablo Clone) because he generates very fast. Crushing blow is very useful for certain builds like the Dragon Talon Kicksin, and a must for builds like Blade Fury Sin since their base damage is very low but the attack is extremely fast.
2. You cube high runes by putting 3 or 2 of them in the cube together with a gem. Up until Thul, you just need 3 runes, after Thul you also need a gem. Here are all the recipes:
3 Thul Runes + 1 Chipped Topaz = Amn Rune
3 Amn Runes + 1 Chipped Amethyst = Sol Rune
3 Sol Runes + 1 Chipped Sapphire = Shael Rune
3 Shael Runes + 1 Chipped Ruby = Dol Rune
3 Dol Runes + 1 Chipped Emerald = Hel Rune
3 Hel Runes + 1 Chipped Diamond = Io Rune
3 Io Runes + 1 Flawed Topaz = Lum Rune
3 Lum Runes + 1 Flawed Amethyst = Ko Rune
3 Ko Runes + 1 Flawed Sapphire = Fal Rune
3 Fal Runes + 1 Flawed Ruby = Lem Rune
3 Lem Runes + 1 Flawed Emerald = Pul Rune
2 Pul Runes + 1 Flawed Diamond = Um Rune
2 Um Runes + 1 Topaz = Mal Rune
2 Mal Runes + 1 Amethyst = Ist Rune
2 Ist Runes + 1 Sapphire = Gul Rune
2 Gul Runes + 1 Ruby = Vex Rune
2 Vex Runes + 1 Emerald = Ohm Rune
2 Ohm Runes + 1 Diamond = Lo Rune
2 Lo Runes + 1 Flawless Topaz = Sur Rune
2 Sur Runes + 1 Flawless Amethyst = Ber Rune
2 Ber Runes + 1 Flawless Sapphire = Jah Rune
2 Jah Runes + 1 Flawless Ruby = Cham Rune
2 Cham Runes + 1 Flawless Emerald = Zod Rune
3. Moltovir is correct, it does not work if you play on Bnet.