Just quickly going down the list is redolent of the old days and the game's classic form:
bullywug and bullett. carrion crawler. demons, devils, and even angels (none of that Second Edition hapless evasiveness here). ankheg and basilisk and beholder; cockatrice, chimera, cloaker. the classic giants and dragons (including that old favorite the dragon turtle, as well as the hydra) and golems. the behir and displacer beast. doppelganger. shriekers and violet fungi, gorgon and harpy and hellhound. the invisible stalker and the five classic weres, kuo-toa and mindflayer, manticore and owl bear. gelatinous cube and piercer and mimic and roper (which between them killed far more characters than you'd expect). Even the little-used peryton is here, along with the mighty purple worm. all the classic undead. the remorhaz and behir, roper and salamander, shambling mound and sphinx (trimmed from four to just the two, in this case an improvement), and, iconic of iconics, the rust monster.
some come from the letter days of 1st edition (i.e., the FIEND FOLIO and MONSTER MANUAL II), such as the ettercap and galeb duhr, the hook horror and a few others.
About the only true classics I noticed missing were yellow mold and the green slime.
There were a smattering of third edition and 3.5 monsters, but luckily the book is overwhelmingly (say, 90%) dominated by monsters from its glory days.
So, while the new Firth Edition PLAYER's HANDBOOK is strongly reminiscent of third edition in the way it lays out character classes, races, et al., the new MONSTER MANUAL is very much aimed at re-creating a first edition milieu. Just flipping over pages made me want to play.
At this point, no telling what era the DMG (due out in December) will hearken back to. Will it split the difference and take second edition (a.k.a. first edition lite) as its model? Will it, horror of horrors, try to recapture the look and feel of fourth edition? Or maybe it'll truly be something new and, for the first time, Fifth-Edition-y? Time will tell.
current reading: THE BROTHERS CABAL by Jonathan Howard ; TOLKIEN IN PAWNEELAND by Echo-Hawk