Growth of power. It's a particular problem for any long-running comic or story, and really shouldn't be: We should be glad that the heroes are moving up in the world a bit. However, not when the protagonist starts always having the perfect power for a situation.
The local grouchy wizard isn't particularly powerful, but has enough talent to be able to eke out a living helping villagers with their petty problems. He'd probably take it better if half the men weren't always asking for spells to enlarge their manhood. After a while as a gag comic, the writer decides to add a bit more plot, and so begins to add more quirky, eccentric characters to the mix: relatives, friends, and a romantic interest for our wizard, all of which have flaws that keep them as underdogs. The wizard's magic, low-level as it is, begins to find a certain amount of use in helping and protecting his friends.
So far, so good. It's an interesting story, and if the characters are good, it'll be fun to read. However, problems could well arise...
These side adventures lead the author in a new direction, and, having played out the village, he conspires to have the wizard thrown out of it.
Actually, this may be a very good idea. Forcing your characters to uproot allows new plots: We can see how the wizard struggles without his source of income. However, all is not well if, despite the repeated references to our wizard being poor, we never see him work or get paid again.
The plots begin getting more complex, and so the wizard's power "has" to increase to let him battle the new challenges. His magic begins becoming rather more impressive and far more useful with little explanation why.
...No, no, no. You CAN make him more powerful, but have to make it fairly. Either let him study to prepare for a challenge or other "fair" way of increasing power, let him learn to use his old power more effectively [e.g. whereas before it took him quite a lot of time to prepare a large sphere of silence to allow the group to slip into hostile territory, now he just puts it on the soles of their shoes and the hinges of ny doors they come across.], or have all this practice slightly increase his power. If the only way to resolve the plot is to give one or more of your characters a rather major and unprecedented new power, then:
A. You need to think of another way of resolving the plot,
B. They're going to fail, then. Institute damage limitation by them with what power they have, and start a new plot to deal with the consequences of that failure,
C. Set things up in advance. Well in advance and subtly: The readers can see an undisguised McGuffin a mile off. (It is, of course, acceptable to surprise the reader, so long as theey can go back and see the hints.)
However... there's more problems ahead for our heroes...
In an exciting plot, an important and well-liked secondary character manages to overcome a major weakness that was almost defining to his character due to how much it limited him.
Well... yes. That could be a good plot, and done well, I'll be cheering for him. However, it becomes a major problem if this leads to flawscrubbing: The removal of all weaknesses from a character. By all means, let this open up new pathways for him, allow him to do new things he couldn't before - but leave him some flaws and things he can't do.
Also, realise it's going to take him some time to get used to his new freedom: He may well keep thinking he has the weakness, and use the techniques he developed to work around it for some time. Indeed, there could be further problems: I didn't have the glasses I needed whilst growing up until I had been rather badly nearsighted for some time. Even now, over a decade later, I have to think about it to pay much attention to the more distant scenery that always used to be blurry, and tend to walk paying attention mostly to the ground 15' in front of me until hazards like roads mean I have to look up and look around.
The wizard begins to get more and more powerful, and the secondary cast become more and more dependant on him.
Ooh. Bad. Bad bad bad! No character should be the centre of a universe.
The rest of the secondary characters are either flawscrubbed or disappear from the comic. Everything now centres around the hero or the most important secondary character, the villains now become either very powerful politician types or demons and other superbeings themselves, with the loss of all grey areas, and any plot snippet generally runs as follows:
Character tries to do thing himself, manages to some extent, then calls on the now godly main character to finish things off.
By the time it reaches this stage, the comic is unsalvagable. Don't make your character godly. It's never a good idea.
Here are some things to avoid:
1. "The Chosen One": Suddenly, your ordinary character is the only one that can save the world, and he'll need to work hard to get the incredible amount of power he needs to save it. It's that last clause that causes the problem. This storyline is going to ramp your character up to cosmic power, able to save the world from the worst threats. So... what now? The world's in trouble EVERY BLOODY WEEK? I mean, bet with all that power any plot with anything less'd be a sinch. Also, you've just pushed one character far above all the others, demoting them from his equals or near-equals into groupies.
Please don't go on to raise each secondary character to cosmic power in turn. That's just painful.
2. The stupid weakness: He may be all-powerful, but the colour yellow makes him useless. Er, unless he tries really hard to overcome it. ...I'd say that noone'd be quite that stupid and transparent about their false-dramatic plot device, except that it exists. This just changes "The solution to every secondary character's problem is the hero" to "Almost every storyline culminates in the hero either making a great effort and overcoming his weakness briefly, or the secondary characters coming along and pulling the weakness away so he can finish up." Please don't make your plot arcs all identical.
3. The Lord giveth, and the Lord taketh away: Whilst he has any power he should need, it's immediately forgotten about after the story. Again, results in every plotline being the same: Everyone mucks about, then the hero manifests the ideal power to save the day.
4. And the angel came unto Joseph, and said unto him, "UP UP DOWN DOWN LEFT RIGHT LEFT RIGHT B A SELECT START" Possibly the worst of all: The character has some sort of mystical experience or meets a supernatural being, and suddenly gets one or more new talents. Only time this works is when the information given is really MINOR, e.g. the location of a keyhole, or if your name is Diana Wynne Jones (who seems to be able to take anything that's normally a hallmark of an awful story and somehow find the hidden potential in it to make pure gold).
This might *JUST* work for languages, as, let's face it, it's no stupider than any other way heroes commonly learn them. And, of course, if the character is a robot or automata, this might be more valid, as long as you don't go overboard.
In short, by all means, let your characters grow more powerful, but let us see their struggles to become it. We'll cheer all the more when they finally get a bit of recognition. But don't go too far: You'll either force all your stories to be samey, sap out the life of everyone but your protagonist, create a rampaging Gary-Stu out of what used to be a rather interesting character, or, most likely, all three.
Remember: everyone likes the underdog. If you want to keep them the underdog, don't try and kid us by saying that one character is demonstratably the most powerful person on the planet, but he still has all these problems because.... er... Well, because he stupidly doesn't use his powers expept where plot convenient, I guess.