This may get a little long winded, apologies in advance.
The main problem with running plug-and-play/drop-in LED bulbs in halogen housings actually has little to do with proper aiming (although that can be a contributing factor) but is mostly caused by the incompatibility of the light source with the reflector. Halogen reflectors are specifically engineered to direct light emitted by a very small, cylindrical-shaped filament, which is emitting light from all sides. As opposed to a bulky square LED chip, which emits light in only one primary direction. To mitigate this, the designers of these bulbs must place two (or more) LED chips back-to-back in an attempt to simulate the 360 degree pattern of the halogen filament. Unfortunately there is no way to accomplish this without the end result being much bigger than the halogen filament, which means the light source is no longer in the correct place in the reflector, which is what causes the light to be thrown around willy-nilly and into the eyes of oncoming traffic. Add to that also the fact that halogen reflectors are designed to allow a small amount of light above the low-beam cutoff (to light up street signs etc.) and when you put an LED bulb in the housing that is many times brighter than the original, that light above the cutoff is also substantially brighter.
Some of the more expensive LED replacements can sometimes work better than others, but I have yet to see any that actually work well. The other problem that occurs when the beam pattern is sloppy like that is you get a lot of foreground lighting on the road directly in front of your vehicle, which makes things seem a lot brighter but can actually hamper your night vision making it hard for you to see farther ahead.
Just some things I have learned over the years, if you want to really learn about lighting optics and such check out the HIDplanet forums.
