The oft-quoted dictum "passive in form, active in meaning" is utterly nonsensical from a linguistic standpoint (and many would argue too, from a pedagogical standpoint) for describing most so-called "deponent" verbs (there do exist exceptions; these will be outlined later). The saying "middle in form, middle in meaning" captures the truth with the same succinctness; these are not "deponent" verbs: they are media tantum (middle-only) verbs.
The evolution of these media tantum verbs may at first seem perplexing: Latin has an active-passive opposition in voice; there exists no middle. However, it largely appears to be the case that these media tantum verbs arose from Proto-Indo-European middle verbs. There do exist some verbs whose origins remain murky, though. The so-called 'semi-deponents', suppletive in the perfect, are one such good example.
It is also worth noting that Late Latin saw a subsequent rise in "deponent coinages" -- which can technically be considered "true deponents" -- followed by an equally quick regularization of these true deponents and media tantum verbs towards active morphologies.
There are seven broad semantic categories into which the middle can be divided for the purpose of classifying Latin's media tantum verbs (and for affirming that these verbs are, in fact, semantically middle), those being:
- Direct Reflexive: an event in which the participant performs an action upon themselves. This semantic category can be expressed with the reflexive marker sē, but there do exist verbs in the middle under several sub-categories, typically "body action middles": grooming, bodily movement, etc.
- ornor - to adorn (oneself)
- perluor - to bathe (oneself)
- Indirect Reflexive: an event in which the participant performs an action for their own benefit. Similarly to the direct reflexive, this semantic category could be expressed with the reflexive marker sibi, but there do again exist verbs in the middle under this category.
- liceor - to acquire (for oneself) by bidding
- apiscor - to get, acquire (for oneself)
- potior - to get possession of (for oneself)
- Naturally Reciprocal Events: reciprocal events in which participant A is performing an action on participant B, and participant B is performing the same action on participant A; i.e, events which naturally involve reciprocity.
- osculor - to kiss
- conflictor - to fight
- amplector - to embrace
- luctor - to wrestle
- altercor - to wrangle
- copulor - to be joined
- Collective: an event which is similar to a naturally reciprocal event, but the action is instead carried out by the participants as a whole; i.e, the participants are not highly distinguished from each other.
- misceor - to mix
- congregor - to assemble, congregate
- colloquor - to converse, discuss
Chaining: an action in which participant A acts on participant B, B on participant C, C on participant D, D on participant E, and so forth. There are very few verbs which encode this meaning, save for the obvious exception: sequor - to follow.
The Cognitive Middle: a fairly broad category that can be tersely summed up as a mental event in which the subject is both the initiator and the affected; this category can be split into several further sub-categories.
- misereor - to feel pity, pity
- vereor - to respect, revere; fear, dread
- meditor - to think, reflect upon
- interpretor - to explain, expound
- comminiscor - to devise, contrive
- polliceor - to promise
- Spontaneous Process: an event in which one subject undergoes a change of state with no specified agent; the subject is the nominal participant.
- morior - to die
- scindor - to tear, split
- nascor - to be born
To summarize all of the above:
- The concept of 'deponency' and the term 'deponent' are not useful for explaining Latin verbs that exist without active morphologies; they are better referred to as middle-only or media tantum verbs.
- The Latin media tantum verbs are largely derived from historically middle Proto-Indo-European verbs; semi-deponent verbs, suppletive in the passive, represent the main exception.
- There do exist "deponent coinages" -- verbs which can truly be considered deponent -- which exist outside the class of media tantum.
- The media tantum verbs can be divided into seven broad semantic categories, which themselves clarify why the classification of middle makes the most logical sense.
The bulk of this answer was largely paraphrased from Seumas Macdonald's excellent piece on the Latin (and Greek) middle voice: Reconceiving the middle voice for Greek and Latin students, itself largely based on Suzanne Kemmer's work The Middle Voice and Rutger J. Allen's dissertation The Middle Voice in Ancient Greek. A study in Polysemy.
While I believe that I have, for the most part, given an effective summary of the work he has done, I highly recommend that you read his original work. A PDF of it can be found here; the original blog posts from which the piece was compiled can also be read on his website The Patrologist.
An interesting aside: the accurate re-classification of so-called 'deponents' as media tantum verbs is not recent; from George Choiroboskos' 9th century commentaries on the canons of Theodosius of Alexandria:
"οὐδεὶς γὰρ λέγει ἔρχω ἢ εὔχω ἢ πέτω ἢ δέχω ἢ ὀρχῶ καὶ τὰ λοιπά, ἐπειδὴ τὸ σημαινόμενον κωλύει."
"For no one says I 'go', or I 'pray', or I 'fly', or I 'receive', or I 'depart' and the rest, since the meaning prohibits it."