简直, 几乎, 恨不得 — overstating the degree

规则 几乎 = almost — it measures the gap and could be quantified: 几乎所有人都来了。
简直 = simply, downright — an overstatement carrying feeling: 这简直太好了！
恨不得 = could hardly wait to — wishing for something unreasonable out of impatience: 我恨不得马上飞过去。
简直 does not measure: 几乎一样 states a fact, 简直一样 is an exclamation.