What are the major differences between version I and II of Canon 24mm tilt shift lens? Right now, used version I is half the price of II.
The biggest operational difference is that version II has a super-rotator style design that allows the tilt and shift axis to be varied at any point relative to each other. Version I allowed the tilt and shift axes to be either aligned or set at 90 degrees to each other, and changing between the two settings required partially dismantling the lens! With version II there's just a knob to control the relative angles of tilt and shift.
The other main difference is the optics, version one of the 24mm tilt shift is 22 years old now and the new version was designed for digital and features a more sophisticated design (16 elements vs 11), with modern coatings which bring a host of optical improvements. Extreme corners feature much improved contrast and sharpness, with less lateral chromatic aberration.
Alongside corner improvements the image circle is significantly larger (67.2mm compared to 58.6mm) allowing greater shifts and reduced vignetting. Maximum tilt is increased marginally to 8.5 degrees from 8. There is also a switch on the lens to lock tilt in the neutral position if you only want to shift.
You'll notice the differences if you do a lot of shifts. If you're mostly using the tilt function then version I is a good buy, though you might want to consider the Samyang 24mm tilt-shift which is a similar price but offers a super-rotator design. Performance is pretty bad wide open, but if you intend on using the lens to extend depth of field then it may perform better than the old Canon in the corners at f/8.
The most significant difference is the ability of the TS-E 24mm f/3.5 L II to do TS rotation. This means that the relationship between the tilt axis and the shift axis can be altered from between 90º and 0º and any point in between. This can be accomplished on the fly during normal use of the lens. The original TS-E 24mm f/3.5 L can only be set at either 90º (standard orientation when the lens comes out of the factory) or 0º between the tilt and shift axes, and requires that the lens be partially disassembled to change the angle. This feature vastly expands the possibilities of how tilt and shift may both be used in combination to affect the perspective of an image. If you are interested in taking photos that only require either tilt or shift, but not both, then this feature is not much of a factor. If, on the other hand, you are interesting in discovering the full creative possibilities of a perspective control lens, then it is revolutionary.
As the linked review above relates, the "II" version is also superior optically, especially in terms of sharpness and lack of chromatic aberration. If you look at the sample images the review provides, you can see that the differences are more than trivial - they are rather significant.