So, on Sunday night for the Thousand Thrones campaign (session 3), a couple of people couldn't come, right at the first session where you really had to be there for (the ship we were on was sinking). So the PCs who were there grabbed the character sheets of those missing, and did our best to get them out alive along with everyone else. Along with my Waldemar, my celestial wizard apprentice, I ran Otto the Pit Fighter because his player was out sick. I wrote Waldemar's character sheet out by hand, as I do with almost all of my character sheets no matter the system, but Otto's player had used a preprinted character sheet. This made finding things unnecessarily complicated, and I encourage all of you to stop using them.
One of the many advantages of doing your own character sheet up with a piece of printer or lined paper is that you can make it as complex as you want, which is usually not very complex at all. As your character grows in power, you can add onto it, or maybe rewrite it out so that the formatting is a bit better when your lists of powers start to run into one another. e.g. My wizard's character sheet only lists the skills I have and the talents I have, and I can tell if I do or don't have these things simply by glancing at it.
But a preprinted character sheet has to accommodate all the complexity and variety of the game from the get go. So usually every skill is written on it, and there's a section set aside for "Spells" even though only one person casts them, or in D&D there's always a box for alignment even if you're playing without alignment, and so on. This makes the sheet overwhelming. I can see the two new folks playing Thousand Thrones with us hunting around their sheets trying to find things, and I know that when I was using Otto's sheet (which is the same format as theirs), I couldn't find anything I wanted to, despite being fairly familiar with the rules to WFRP 2e.
Exacerbating this problem is that it never seems like anyone with the skills at laying out documents knows the rules to the game very well. This is most apparent in the miniscule size of spell sections on D&D character sheets, or in the almost hilariously small areas set aside for talents in Dark Heresy character sheets, but similar kinds of problems occur on almost every premade character sheet I've ever seen.
I also think these sheets encourage poor notation. All you have room to write down under the "Spells" section is the name of a spell, whereas a proper wizard player should actually have the mechanical details of all their spells written down for rapid reference so they don't have to hog the book every time they want to cast something or decide which of two spells is more useful or resolve what happens. As you learn more spells, you also write them down, and when you level up, you modify the spells appropriately as part of the process, so that it's very manageable and piecemeal. It also reinforces your own memory of what the spells do, so that you start to remember that Dimension Door in 3.5 doesn't have somatic components and can therefore be used to teleport out of your bonds or be cast while wearing plate armour without a chance of spell failure, thereby building the kind of system mastery that distinguishes the future-archmage from the bloody smear in robes.
Typically, when I run a D&D PC, I use three sheets of paper. One is just stats, one is gear, and one is spells (I almost always play wizards), since those two things eat up the most space, require the most revision, and are often consulted separately from the rest ("Do I still have the chisel?" should not require you to scan past tons of calculations). A separate gear sheet is useful since it lets you organise your gear into sets which your PC can switch between.
For example, Siegfried Hausmann, my limping, atheist doctor with a shotgun in Economy of Force (another campaign the Liber Fanatica guys are playtesting, currently on hiatus until Thousand Thrones is over) has a "town" set of gear, an "adventuring" set of gear, and a total listing of all gear he owns, so that I can say "I put on my good clothes and go to town" without needing to figure out thirty minutes later whether that means that I've brought my medical tools (Answer: Yes). Preprinted character sheets suppress this kind of organisation and present the lowest common denominator in their place.
And finally, the weird spacing issues. Borders, wasted space, tiny spaces between the lines that you would have to have needles to write comfortably between, all come together to make reading and writing on a preprinted sheet completely horrible just as a document. I wish I could slap everyone who ever put the name of the game on the character sheet, since it's usually the largest and most prominent piece of text, just in case you forget you're playing DUNGEONS AND DRAGONS 3.5 or WARHAMMER FANTASY ROLEPLAY or whatever because someone has concussed you with a handful of dice. Tiny writing is writing it's hard to read, and hard to read means hard to find and slow to reference, and since allowing the easy reference and retrieval of information is the point of the sheet, it directly impacts the value of the character sheet, pushing it closer and closer to "active impediment".