Something that would be pretty neat IMO, and possibly work to deal with (at least partially) the objections that you can't tell a bunch of monsters with the same height/strength description apart:
For Fortress mode: The bookkeeper can, as a part of his normal job, keep track of information on the creatures you've fought or had a chance to observe. At first, the only information you'll have is what you would gather from simply looking at it (size and appearance). With time, however, either because you fight the creature (goblins, kobolds) or because you get more chances to watch it, your information becomes more detailed. Fighting the creature gives you information on what kind of attacks it uses and how strong said attacks are, as well as natural armor and basically how dangerous it is in general to fight. Simply watching the creature gives you information on how it behaves, perhaps if it is intelligent or not (for the monsters, perhaps, that your originating civ never had contact with) and if it uses equipment, and also what time of the day it is active. Fighting creatures firsthand will give a vivid but not necessarily accurate account (within reasonable limits, obviously), while collecting information from other fighters will result in a more generalized and averaged account. Fighting a single type of creature more often in either case results in a more accurate and exacting description (though it won't necessarily be represented in numbers). This information will be kept in a logbook, which will be taken with you on embarking but which can also be destroyed during the course of your fortress' life (in which case, you will have to make a new one, and in which case your monster/fort information may suffer a setback).
For Adventure mode: Your fighting experiences are kept in your head and, as in Fortress mode, you get a better sense of how strong monsters (and you) are as you encounter and fight more creatures. You can also go to sages, who will sell you information on specific monsters, as well as read logbooks from dead fortresses (though depending on how damaged they are they may be more or less legible) to find out more information on monsters. Also, a new type of quest (from dwarves, no less) could be to head over to dead forts and retrieve the logbook from within so they can see how their fellow dwarves perished.
This is sort of a takeoff on a feature known as monster memory, which I think can be adapted to fit DF nicely.