Mysticism as a skill in the past was my favorite magic school, and to be honest, I think removing it and throwing its spells into other schools was a mistake. Most if not all of the spells (including ones from past games, not just Oblivion) could technically be put into other schools without much contradiction (Soul trap makes some sense in Conjuration or Enchant, telekinesis in Illusion/alteration, etc) but I think those effects worked best under a mysticism school, especially in the case of teleportation spells such as Mark/Recall and Intervention, which were removed in Oblivion due to near pure laziness. (it wasn't hard to designate areas and certain times when teleportation is disabled, as was the case in Morrowind, thus preventing the breaking of scripting, which was to my knowledge the only issue. Or at least the main one)
And this is because just going by real world definitions of Mysticism, the spells that used to fall under it fit best under that overall title. (yes, I know that schools really don't matter in the magic of TES, but this is beside the point) Mysticism, as it is defined, deals with spirituality and the essential fabric of reality and unreality, life and death,. (as I understand it and can paraphrase it anyway) Hence spells like Soul Trap (connection with the inner soul, taken to the extreme of being able to connect with other souls and entrap them; is perfectly reasonable in a world of magic), Intervention (deep intrinsic connection with reality and god(s). Taken again by magic to an extreme, thus enabling teleportation), reflect (a possible extreme of self-nullification), absorb (see reflect), and finally detect spells (connection with reality, taken to extreme and specifics) fit best under a Mysticism school because their inherent connection to what mysticism is, both in the Elder Scrolls and in real life. The dispel spell is the only spell that isn't necessarily connected to something related to real life mysticism, but generally it doesn't really fall under any other school except possibly restoration, but that is pushing it.
Now, as far as gameplay goes, I've seen very many people say mysticism was useless, despite the fact that virtually everyone made use of Soul Trap in Morrowind and Oblivion, and many would have probably used Reflect and absorb spells, despite the fact in Oblivion that absorb spells fell under Restoration. (a dumb move) And obviously everyone in Morrowind used mark/recall and the two intervention spells. Dispel was obviously very useful. Telekinesis was the only mysticism spell that was really and truly useless through out the games, and this is only because there was never any reason to use it. Never did we see some sort of impassable chasm at the other side of which resides an object that only telekinesis could allow you to reach or some other situation that called for telekinesis.
And this even has some lore effects as well, as the Psijics are among the main students of Mysticism, and having it removed kind of eliminates not only one of their unique qualities, but also makes them into just a glorified Mages Guild of sorts, seeing as they now practice spells from all other schools.
AS far as Skyrim goes, I can understand consolidating many of the skills that they did, especially speechcraft and mercantile (though depending on how dialogue works, they can be separated again) and acrobatics and athletics (this one goes without saying), but I do not think it was a good move getting rid of mysticism, as it only seems to have happened not because it was useless, because it most certainly wasn't (may be in Oblivion, where half of its spells were removed or still useless), but because if they didn't magic would have one more skill than the other two specializations. And quite frankly, I do not know why thats a problem. Sure it makes it so that mages will inherently be more powerful level wise than the other specializations assuming all skills related to it are trained, but as is the case with all elder scrolls games, and certainly Skyrim, in the end you're either powerful enough to hold your own against most anything regardless or exactly the same anyway. (having trained everything)
Having equal amounts of skills across the specializations is just plain boring in my opinion, and really only made sense as far as the attribute system goes, but seeing as we are going to see that being taken out (good riddance) I fail to see why it still matters except in a rather pointless try at maintaining balance in the skillset.
So what does everyone think?