The way GPH has the joystick, it reads diagonals
too well, and it won't read if you only lightly gesture. The first problem is very apparent in some kinds of games, such as fighting games, but not so much so in others, like platformers. The second problem is apparent if you're for some reason trying to apply only a little pressure, but is still alright. Also, the shape of the thing takes some getting used to, but it's really not all that bad - plus, there's always DaveC's stick.
The scanlines are a pretty big problem. They
will bug you, alot at first. With the LCD/clock speed changer you can help alot, but they are still going to be there. They should be fixed with a firmware update, but until then, they'll probably be your #1 problem.
I believe autobricking only happens when you have a compatable SD card formatted with an ext2 filesystem, and if it does in other cases... well, you have that way if you want to try for yourself. It's a major bitch, but any warranties should cover autobricking, and there are ways to unbrick. I wouldn't worry about it though - it's only happened to a select few.
There is one more problem you didn't mention, which I guess I should warn you about. In the first batch of units, alot of the headphone sockets weren't soldered properly and sunk in pretty quickly. I believe some later batches suffer from this, though not nearly as much. It's, again, a problem that your warranty should cover, and there are plenty of, apparently easy, guides to doing it yourself if you have to.
All things considered, the problems don't amount to that much. The joystick is a problem that you'll get used to, the scanlines are most likely a soon-to-be-fixed firmware bug, autobricking is rare, and so is the headphone jack falling in... And even in the worst case scenario, you
can fix the thing yourself with minimal effort.
I hope that was enough. Good luck.