the differences between the qc code is that qw mods are not generally allowed to affect the player's movement (jump heights, swim speed, etc) without asking the engine to do it for it (ie: .gravity and .maxspeed instead of dumping code in playerpostthink). This is because the client needs to match, and if it doesn't then you get juddery movement/jumps.
qw qc code avoids the use of MSG_BROADCAST. Its available, but its better to use MSG_MULTICAST (really though, you're better off using the proper extension builtin instead of directly using WriteByte).
qw qc code doesn't support particle(). there are some replacement TE_foo effects for commonly used effects, but that's pretty much just blood.
qw's te_gunshot has a count argument, resulting in a single te_foo in quakeworld where you'd have 12 separate bandwidth suckers in nq.
qw qc code permits multiple thinks on a single entity in a single frame, keeping timing more correct even with an overburdened server (other programs stealing cpu etc).
qw qc code runs players out of order (when handling packets received from the client) instead of all ents thinking in entnum order. Note that DP defaults to be more like QW here, its just a sideeffect of prediction.
svc_setview does not exist in quakeworld. spectators are an engine feature and mods do not need to do anything for them.
vanilla quakeworld does not support .spawnflags based spawn filters - it'll always act as if deathmatch 1 is set. custom qw engines have reverted to nq behaviour here.
quakeworld has userinfo stuff, which permits per-user settings and options which are readable by the gamecode. nq depends fully upon settings stored on the server.
most qw mods do not support single player (see .spawnflags comment). the server still supports all the builtins the ai uses, just that most mods lack monsters and thus lack ai too. quakeworld's traditional only-dedicated-servers stance kinda removes the need for single player support in mods.
for a full list of the differences between nq and qw (and h2) gamecode, see FTE's progstype variable.
in general the changes required to convert a mod from nq to qw equate to: replacing defs.qc for the qw systemdefs/systemfields/builtins, removing the .velocity changes in client.qc, particle() -> te_[lightning]blood, .punchangles->svc_big/smallkick, EF_MUZZLEFLASH->svc_muzzleflash, te_gunshot -> te_gunshot+writebyte(count), bprint(msg)->bprint(PRINT_HIGH,msg), svc_cdtrack changes, svc_intermission changes.
certain mods have additional dependancies (eg: svc_setview), but the above should cover vanilla qc.
for much of it you can use #ifdefs with fteqcc or frikqcc and write a mod that directly supports either.