I've done a lot of research in whips, both in real life and in D&D. Basically, the normal whip is too weak.
It is a one-handed melee weapon, but it does d2 damage, compared to the d6 that is the minimum for all other one-handed melee weapons. It can only do non-lethal damage, and even then, it can only hurt people not wearing armor. It has a reach of 15, but it doesn't threaten an area, and even using it provokes attacks of opportunity. True, it grants a bonus to disarm, and it can be used to trip, but these are minor. Finally, it requires an exotic weapon proficiency to use, and while the whip is indeed really frikkin' hard to use as a weapon, it is too weak as written.
The whip needs to be improved, but some things we cannot change. I can accept a whip not being able to threaten an area. Whips are hard to use reactively, especially against someone charging at you. That might be fair as some high-level feat, but not at low levels.
It also is fair that a whip would only do nonlethal damage. While whips can tear the skin of cows, they simply cannot physically puncture the body and damage an organ. Perhaps a feat could allow whips to deal lethal damage, requiring the use of exotic manuevers, like grabbing your foe's throat and strangling, or grabbing their arm and yanking it out of joint. But normal whip usage makes sense to be nonlethal. Perhaps a whip could be a martial weapon normally, and having exotic weapon proficiency makes it deal real damage.
Range is actually a factor where whips are a little unrealistic. A typical bullwhip is between 8 and 12 feet long, with some as short as 6 feet. If a 5-ft. long greatsword threatens only 5 ft., and a 10-ft. long spear threatens 10 ft., maybe whips should not go beyond 10 ft. There are some whips, used from horseback, that are 20 ft. long or longer. But we can stay with the 15-ft. reach.
A shuriken, which is a tiny piece of metal with a blade maybe three inches long at most, can hurt someone in full plate. A scimitar, which has very little force behind its attacks, and is basically just a cutting edge, can hurt someone in full plate. But a whip for some reason cannot hurt someone wearing leather armor. If the rules make allowances for these other weapons, they ought to do the same for whips.
Is d2 damage appropriate? Too little? Well, being clipped by a whip isn't anywhere as bad as being chopped with an axe, but maybe it's about the same as being slashed with a knife. Make it d4?
What about provoking an attack of opportunity? Yeah, I suppose it makes sense.
So, for a balanced whip, we can have this version:
Whip: d4 damage, reach 15 ft., does not threaten an area, provokes attack of opportunity when used, +2 bonus for disarm, can be used for trips, does normal damage.
The other abilities we might want to provide can be accessed with feats.
By the way, did you know that you can power attack with a whip now?