Every student has experienced a teacher whose lectures are incomprehensible or mind-numbing. As a student of Physics at the University of Illinois at Urbana-Champaign I frequently overhear and participate in conversations about the teaching methods used by different teachers and courses. The following is based on those conversations — mostly with other students in the sciences.
One advantage of chalk is that teachers often do not properly utilize technology. The slower pacing that is natural to chalkboard lectures forces teachers to add detail to and expand on the roles of each piece of a fact/equation/diagram. They are unlikely to miss any details since they physically proceed through each step of a process. Using a PowerPoint presentation would, in theory, allow more time and energy to be devoted to explanations and answering questions but, in practice, that is not how technology is utilized. Teachers often step from wall of text to wall of equations to wall of equations to wall of equations… and get ahead of schedule only to answer questions that would not need to be answered in the thorough approach naturally forced by traditional chalkboard lectures.
In addition, the reiteration from the teacher’s notes or memory is another chance to catch and correct the inevitable mistakes (which may be abundant depending on the teacher). It might be the case that mistakes are less frequent if the lectures are carefully prepared, however. It can also be very helpful to students to see how teachers draw diagrams and use notation in real-time. Animations of complex processes can be invaluable in understanding some material, but those animations are better utilized as supplements to chalk lectures, in general. On top of that, it is easier for an instructor to change the course of a lecture, incorporating student feedback, with a chalkboard lecture — for obvious reasons. That is perhaps the most significant difference, and even that can be compensated for in a carefully constructed pre-prepared presentation.
Ultimately, the benefit/detriment of technology in the classroom depends entirely on the ability of the teacher to integrate it such that it is not distracting, boring, unpolished, or (especially) being used as a crutch. I concede that if carefully utilized and well-thought out, technology can improve the classroom experience. It also has the advantage that it can afford those with a variety of physical disabilities to lecture and teach unhindered by those disabilities.