Here are all the problems with Swashbuckler as a class:
1) They can easily get out-attacked by someone wielding a sword and shield because of specialize, fury, etc.
2) They can easily get out-damaged by someone in the front row also doing the same amount of attacks; since their prime stat is dexterity, they are a surprisingly poor choice for a human.
3) Haste very rarely gives them a fifth attack. Very, very rarely.
4) Since they do not wear heavy armor, they tank roughly as well as someone in the front row, so they can never be aggressive without almost getting killed.
5) Massive weapons are insanely difficult to parry; in fact, there's way too many weapons that are too difficult/impossible to parry now (they at least get a chance VS some of these weapons, though).
6) Good light armor is difficult to come by, and the best light armor available breaks too easily to the best weapon materials available.
7) The best weapon(s) for a swashbuckler are also the best weapons for other front line classes (epee, rapier, kama, sai, etc.), and other classes can put those weapons to better use than they can.
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Mithril is one of the most surprisingly disappointing materials in the game, and supernatural armor no longer exists at all.
9) Dragonscale would need magical vestments up all the time, so it would need a consumable of some sort or a scripted item. Dragonscale is also surprisingly disappointing.
10) Despite there being a cloth-equivalent set of equipment that usually compares equally to adamantite (energy), there is no such set of equipment for light armor.
11) Mithril armor can lose all its protection when damaged enough, while heavy armor simply will not.
12) There is a surprisingly low amount of good mithril equipment in the game, while there is a wealth of good adamantite and diamond. Furthermore, there is no "diamond" for mithril (same protection with some drawbacks) like there is for adamantite.
13) Swashbucklers have no way to immobilize someone, so they can't keep up with other classes that can quaff, because fly is way too good (even with what I think have been some nerfs already?).
14) There is roughly one swashbuckler build that is anywhere near competitive.
15) Swashbucklers are THE most passive class in the game: they do practically nothing active; and even if they could, they'd spend most of the time not doing it because they won't be able to keep most enemies immobilized. (This is more of a problem with immobilization options, though.)
Most of these are extremely good points and summarize the swashie drawbacks pretty well. A few of them are a code/building issue, though. I will refute the statement that there's a low amount of good mithril eq in the game. There's plenty of good mithril, but it's far more difficult to obtain than the addy/diamond. Even some of the energy armor is easier to get than the best mithril.
Having an equivalent for light armor to the cloth energy is a great suggestion, but that only addresses a small part of the problem with swashies. The main problem with having a front line melee class like swash is the lack of immobilization options. Bash > all. Taunt is meh, and situational at best. The lag on a failed taunt is far too long to risk using it in most cases. The lack of an ability to prevent quaffing, while simultaneously being prevented from quaffing is really harsh on the class. Trip just doesn't cut it, not when everyone and their brother runs around flying all the time anyways.
The number of attacks they get is also an issue. They can't afford to be aggressive due to armor class and being bash fodder front-row types, and they can't effectively parry many of the weapons used by the other front row classes, whether it's because of accuracy issues due to spec/furied already-high accuracy weapons, massive flags, or no-parry subtypes. That's a problem too. A fix to that could be something like a third parry skill available only to the class which allows them to face things like massive and no-parry weapons as if they were normal types.
Lack of quality copies of subtypes swashies need to be effective is also a problem, since most of the best ones are stacked in the inventories of non-swashies. This is largely an issue with the disarm problems, and despite the recent code change to the success rate of disarm it won't change any time soon. The lack of ability to grab weapons during combat is not going away, and people who fear disarming are still going to carry a billion spare weapons, and many of those people are going to insist that they be the best materials for the subtypes they use. 'Cause, you know, the third weapon you reach for after two disarms has to be able to take a beating. You use it all the time. /sarcasm.