In Diablo II, what's the recommended level before starting Nightmare (or Hell) difficulty?
I'm not sure if theres a real concrete answer to this. Different classes, built with different skills, with different items, played by different people will all function differently. Some characters (Like a minion-necromancer) will be able to handle higher difficulties at lower levels then say a mage (especially in hell when everything has multiple elemental immunities).
Without the expansion, I generally tried to hit nightmare around 35-40 and hell 50-60.
With the expansion, I tried to hit nightmare around 45-55 and hell 65-75. You'll want to be level up even higher if your playing a hard-core character.
Hope this helps
It really depends on how comfortable you are with fighting tougher fights, and whether you are playing HardCore or not.
I personally try and hit Nightmare as early as possible, as you level much faster if you are lower when going in. I generally enter Nightmare around level 30 if possible. I definitely wouldn't wait until 45 or higher as some comments have mentioned.
The earlier the better. It provides more challenge anyways.
Are you playing single player or on battle net?
If it's battle net, I would just recommend getting rushed till a4 hell and then leeching chaos runs to hit whatever level you want. For single player, it depends heavily on the gear/class, but try to have your bread and butter skill maxed (or at least started if its a lev 30 skill) by the time you finish normal and have most of the synergies for it by the time you reach hell. Having high resistance and leech also makes a huge difference.