Allright, I've been looking about at this wiki for approximately one year now - though I only actually joined quite recently; what I've noticed is that while some Zanpakuto are pretty good, some other tend to be, bluntly said, pretty bad; this is usually due to multiple factors - the most common factor is as simple as a lack of originality; some even go as far as to completely copy Zanpakuto from the original cast, most commonly being that of Zangetsu, while I'm perfectly aware that this is possible in the Bleach universe: At least as far as the movie is concerned it simply doesn't give off a good vibe in my opinion, why you might ask?
Well, as I see it, when I see a character whom uses the Zanpakuto of a Canon Character, yes perhaps even the Zanpakuto of the main character; I can't help but try to subconciously avoid that person, unless of course they have a REALLY good backstory and that the character itself is very well-written. A single person whom have the same Zanpakuto as Ichigo would be allright, but three, four five and seven? Not a chance, after all. The case with Hyorinmaru, Kusaka and Hitsugaya was a very rare occurance.
I also wish to remind one that this is a completely personal opinion of how Zanpakuto should be made, and it is in no way the only way to do so.
Zanpakuto and Shinigami
Right, a part which I deem to be one of the most important aspects of the creation of a Zanpakuto is the close link between the weapon itself and its weilder, many Zanpakuto that I've seen don't have a clearly defined link to their owners personality, which I deem to compromise the weapon itself: Most seem to think that aslong as the weapon has cool powers it doesn't matter - its always good to remember that Zanpakuto are physical incarnations of the Shinigamis very soul, and thus their abilities should fit the weilders Personality: Look at Rukia Kuchiki for example, cool, level-headed and inteligent; now lets take a look at her Zanpakuto, it fits perfectly!
There are of course exceptions to this rule, in that at the first glance the Shinigami and the Zanpakuto seem to be completely mismatched, a good example would be Hisagi Shuhei and Kazeshini; while they appear to be completely opposite - they are in the end quite similiar to one another: Buttom line is, wether it is very evident or not, the Zanpakuto should have atleast a single trait which binds it tightly with the Shinigami in question.
A Short Basis on Zanpakutō Types
In the world of Bleach, there are three main types of Zanpakuto, these are Melee, Kido and Elemental types, the first two are very defining, the third one would all be concidered Kido types but they're so varied that they're concidered a type all on their own. As such, Kido Types are generally limited to Projectile, Defense, Poison and Illusion. A very important notice is that Kido-Types don't have to only consist of only Ranged-Abilities, or even have any ranged abilities at all! As an example Wabisuke is in fact a Kido type. Kido types generally have several abilities, though excepions do exist. Also, Kido Types are abilities are usually well varied, though they cannot be expanded upon in most cases and using them also generally requires a good deal of training. (As a note, most Zanpakuto whom people deem to not fit in any of the different types can safely be dumped as a Kido Type. This is much better than to overload the Zanpakuto List with "Sound-Types, Phoenix-Types, Dimension-Types, etc")
Melee types are basically the types that are employed by many recurring characters in the series, with Zangetsu perhaps being the best example: These Zanpakuto are obviously well suited for close combat, possessing many traits and abilities which increase the power of their close-ranged strikes or so forth: What truly separates them from the Kido Type and the Elemental Types are that instead of many special effects, they usually have only one or two, yes - Zangetsu does in fact only give Ichigo a singe ability, Getsuga Tensho. Though Ichigo has learned to use this technique in many different ways to compensate for it.
Elemental types are pretty self-explanatory in that they have several different abilities centered around a single element. They commonly have more than a single ability, though those are usually set-in-stone meaning that they can very rarely be used for anything else than for what they were designated to do. Unlike the Kido types, whom very often have several abilities regardng to different situations.
Controversy Regarding Certain Zanpakutō Types
Right, some Zanpakuto-types are regarded as being unfair to use in roleplays; these are primarily Illusion Type Zanpakuto, whom are generally very powerful, if you look at Aizen's Kyōka Suigetsu it should quite quickly become apparent why this is so - however, there are some Illusion Type Zanpakuto whom are powerful but yet made fairly, the only known example I can come up with would be Jōzai. Which is the Zanpakuto of the fan-made Naibu Shizuka. Though the Zanpakuto itself is made by Koukishi, so well, hats off to him!
Reality-Warping, right, these are possibly even harder to make with balance, though there is one Zanpakutō whom I deem to be made successfully which actually does things like this; Namely Man'yōshū. It is made by Waterkai and he definately desserves praise for its creation!
Finally, and this is what people tend to become agiated by the most; which is Zanpakutō with the ability to copy the abilities of another Zanpakutō and use them on a permanent basis: This is, excuse my blunt imput, very very rude unless you've agreed upon it beforehand: Senior Roleplayers usually spend a lot of time in designing their Zanpakutō to fit the image of the character they're assigned to. One thing is to copy a Kidō spell or even an advanced Hohō or Zanjutsu Technique; quite another is to copy the Zanpakutō; which is by definition the weilders very soul, and thus it shouldn't even be possible!
Copycat Abilities
Right, this is what I personally dislike the most when it comes to certain Zanpakutō; This is the practise of copying one ability from a Canon Zanpakutō, and adding it to your own; yes, in especially dire cases this may also happen with Fanon made Zanpakutō! Some of those whom do this actually changes a good deal of the traits and makes it their own ability, which is fine - as that is called inspiration: Some yet only Copy-paste the designated skill over to their own article with minor changes (Usually simply changing the mentioned names)
Kyōka Suigetsu is currently the one whom is most used in this regard, this is especially bad in my opinion as Kyōka Suigetsu's ability is a god-mod in itself: And in order to avoid being instantly killed, one is forced to resolve to equally god-moding solutions in order to protect ones character. It creates a rather ugly circle and reduces the fun in roleplaying alltogether: Copying certain aspects of Kyōka Suigetsu rather than the whole weapon ain't out of the picture though. Perhaps in contradiction to fooling all five senses, it only fools one at a time? Another reason that this is bad, would be that Aizen is the main villain of Bleach; and his Zanpakutō are widely regarded as the most powerful in the series – to copy its abilities serve to push his character down and remove much of what makes him special. Which is basically downgrading Kubos work.
Even worse are those whom copy the abilities to a Zanpakutō whom already have several abilities; because this too in 7/10 cases make the Zanpakutō in question overpowered and thus unfit for roleplay.
Balance
Zanpakutō are required to be balanced in order for them to be used in Roleplay, most Roleplayers don't want to fight against a character whom can effortlessly counter all of their attacks trough the use of his or her Zanpakutō; not to mention that this too is a form of Godmodding (Yes, there's a lot of things whom count as that) - for more information about that subject I suggest reading this! Every good Zanpakutō needs to be balanced, and by balance I mean that its abilities have flaws, and that it doesn't possess an Ability of every designated type. As an example, and as much as I hate using my own Zanpakutō for reference; Shirohane, while a very powerful Defense-Type Zanpakutō in its own right, has only a single offensive ability; and this ability is very limited in that if it is used unwisely can severely deter Hiroya's lifespan: Moreover, this Zanpakutō's edge is eternally blunt, making it incapable of ever wounding an opponent in battle, making it useless for offense even in its Sealed State. It has another offensive ability too, but that involves using Hiroya himself as a voodoo doll of sorts.
Right, this ends my vain example. The bottom line is, try to keep your Zanpakutō to a single discipline, or if it is an all-round Zanpakutō you have to make sure that it is not really better than average at anything, perhaps with the possibility of strengthening a single aspect of it at varying occasions.
This concludes my short but hopefully somewhat helpful guide!