To quote the 3.5 rulebook:
Because a whip can wrap around an enemy's leg or other limb, you can make a trip attack with it. If you are tripped during your own trip attempt, you can drop the whip to avoid being tripped.
When using a whip, you get a +2 bonus on opposed attack rolls made to disarm an opponent (including the roll to keep from being disarmed if the attack fails).
A 15 foot flank trip attack, where the bad-guy getting up provokes an attack of opportunity from the main fighter is incredibly powerful, even if you do no "damage".
These same rules are in Pathfinder (pgs 143-145). Whips are listed as disarm, nonlethal, trip weapons on the table on page on 143, then from 144-145:
Disarm: When you use a disarm weapon, you get a +2
bonus on Combat Maneuver Checks to disarm an enemy.
Nonlethal: These weapons deal nonlethal damage (see
Chapter 8).
Trip: You can use a trip weapon to make trip attacks. If
you are tripped during your own trip attempt, you can
drop the weapon to avoid being tripped.
Again ... that trip attack is invaluable. The only "more broken" weapon is the spiked chain in 3.5, which does threaten the 10' reach and allows a trip attack, which means you get a free trip attack on bad-guy again when he tries to get up, along with your normal attacks once you trip him (one of my favorite characters ever was a Dwarf Fighter specializing in a spiked chain).
Yes: curses, ability damage, and ability drain causes loss of the ability to use the feat. You don't actually lose the feat, however - you just don't gain it's benefits until you meet the prerequisite again.
From the Players Handbook 3.5 page 87.
Prerequisites
A character cannot use a feat if he or she has lost the prerequisite. For example, if your characters strength drops below 13 because of a ray of enfeeblement spell, he or she can't use the Power Attack feat until the prerequisite is met again
When you lose a feat that is a prerequisite of another feat only the abilities of the prerequisite feat are lost, because you only have to have a prerequisite feat to qualify for another feat and losing the abilities doesn't change the fact that you have the prerequisite feat, you don't actually have to be able to use the prerequisite feat to qualify for the other feat...
...So if your dexterity was dropped below 13 you would lose access to Two-Weapon-Fighting
but you would not lose access to Oversized-Two-Weapon-Fighting.
Best Answer
In theory, a character with a BAB lower than +4 would see a larger AC bonus from Total Defense than from Combat Expertise. Once BAB +4 is attained, Combat Expertise is strictly superior.
However, both options are exceedingly poor. Situations dire enough to give up offensive capability completely, or severely hamper it, should generally be responded to by retreating, not trying to “turtle.” That maneuver is only appropriate when cornered and help will arrive very soon (1 round, maybe 2). Since this is situation every character should be actively trying to avoid as much as possible, Total Defense is superior to Combat Expertise simply because you don’t need to have planned on getting into that situation in order to use it. Because you should be avoiding that situation as much as you can, spending a feat on a situation that occurs rarely and you are actively avoiding is unwise. The attacks you make at a −5 penalty are just not worth a feat, nor is the +1 to AC.
If you are concerned about this situation, and want to spend resources on handling it, it is far, far better to invest in a “panic button” that can get you out of such a situation. Anklets of translocation provide two swift-action, 10-ft. teleports a day for a measly 1,400 gp. That’s worth way less than a feat, and will go a long way to making sure you never have to turtle in the first place.
Unfortunately, that argument is moot in a lot of cases because characters are forced to acquire Combat Expertise because of its frequent use as a feat tax. Once you’ve got it (and BAB +4 or more), there’s no reason not to use it if you find yourself in a situation where you have to turtle. You should still have panic buttons, and should still use them first, of course.