That and something that looks great on paper tends to not be so great in play. I've seen thousands of well thought out interesting PCs enter the game. I bet most of them have max backrounds, the player figured out every bit of past life, all likes, dislikes, fears, loves etc etc etc. And 95% of those PCs are stored or suicided in a couple weeks.
And to be honest, most PCs SHOULD be rather boring to start. Though you might have made them with 40 years of history, the fact is, they do not have that, and it is something easily seen through by other players as being at best a facade.
Even worse is the PC that was well thought out to start, lives a long time and is still exactly the same 6 months later. Yes, he was interesting in the beginning, now, man, boring. People who have everything worked out ahead of time do not normaly [sic] let the PC grow and change. Some might enjoy that style, I do not and will avoid the PCs.
Just wanted to touch on this before offering my own advice.
For the first point, it seems counter-intuitive that a player that puts a lot of effort into a character application would suicide or store their character in a short time. Players regularly lament the loss of their (presumably) well thought-out special applications or sponsored roles after a short time, so there obviously is an expectation that a character's lifespan should be roughly proportional to work put into an app. If your statement is remotely true, I can get behind the idea that these characters are dying accidentally, and you are assuming that they are stored or have committed suicide. That has nothing to do with fun or lack thereof.
As for the second point, either you truly believe PCs should not have a unique personality until they are played for a reasonable amount of time, or, more likely considering your line of argument, you are confusing history
. New characters don't have a lot of skill
(unless staff alters their abilities somehow), and oftentimes history has nothing to do with skill (though the initial background should certainly reflect the starting skills or a lack thereof to some extent). But all characters have some sort of history, and to be frank, it is ridiculous to say that is all a facade because it happened virtually, and I hope that is not what you're saying. It would certainly be odd if someone apps a warrior whose history includes 20 years of them being a master swordsman without any staff assistance, but that is obviously something staff are there to catch. You wrote about your PC with arachnophobia - was that something included in the background, or something developed during play? If it was the former, did you disregard it as a facade?
And to the last, what you're talking about is a static character. How static a character is has absolutely nothing to do with how defined a character is to start... or, put in another way, characters that are well-defined to start and characters that are vaguely-defined to start can all be static. Obviously, you and I both prefer dynamic characters, but the truth is characters with any sort of background can be dynamic. Being able to change a PC over time is really a simple matter, and a PC that exists for 6 months and hasn't experienced anything that could be considered life-changing seems, at least to me, pretty bland.
As for my advice pertaining to the original topic,
I like Marauder Moe's suggestion at the start, as well as what Aaron Goulet and Anaiah's suggestions. Weaknesses are very important to me, as a player. No one is infallible, and it's important to account for that. I like to write about my PC's personality, about things that won't necessarily go into a background but be kept in a place that can be easily changed, for my reference. When I write about my PC's personality, I like to balance perks with flaws - strengths and various things my PC is good at, likes, or likes doing, contrasted with things that they dislike, dislike doing, or are bad at. This essentially helps me check myself and prevent me from making a PC that is good at politicking, AND obsidian-mining, AND singing, etc. Instead, if I decide my PC will be good at singing to start, then they will invariably start off poor at dancing, or painting, or sculpting.