damn.. i had bit more specific lvling guide than djnat's, ill find it for you.
edit
Heres lvling strat for norm: (do this in order) (should be done in 5~8 ppl room)
lvl 1 ~ 6: Den of evil, cold plain, stoney field, dark wood (do cain quest), tristram, and get black marsh wp.
lvl 6~12: Black marsh, Tamoe highland, outer cloister, barrack, jails, inner cloister, cata comb, kill andy (should now be lvl 13 after andy)
lvl 12/13 ~ 18: Sewer runs, get cube and full staff.
lvl 18 ~20: arcane run, kill duriel
lvl 20: kill meph (get some1 to do trav bug thingy... and kill meph)
lvl 20~22: Kill izzy, Kill Dia
lvl 22 ~ 24: Do trav run in full room, try to do this by ur self or with friend if ur weak.
lvl 24: Ancient
lvl 25~42: do baal run (in "quest cannot be completed" room, then kill baal in lvl 42 in full room.)
NM
lvl 42: Kill andy
lvl 42~45: Kill duriel
lvl 45: Kill meph
lvl 45~47: Kill Diablo and izzy
lvl 47: Do Ancient
lvl 48~75: do NM BAAL RUN. same thing as norm.
HELL
lvl 75: kill andy
lvl 75~76/77: kill duriel
lvl 76/77: kill meph
lvl 76/77~78: kill Diablo
lvl 78: Ancient run
lvl 79~99: Baal run!/Mf by ur self(pindle meph in full room)/diablo run (?)/shenk run.. what ever you like.
***lvl 79: Do all the quests u've missed out before/after baal run.
Meh, this is jus rough lvling guide i had for rest of my chars...