http://www.burningsea.com/ Would probably be the easiest example of what you're talking about. They've done a fantastic job recreating Naval tactics. As far as typical MMO's though, I think Dark Age of Camelot would be the closest approximation of what you're talking about.
http://www.darkageofcamelot.com/In DAoC, each realm has different vulnerabilities and resistances to different types of weapons per their armor (Plate=Crush resistant, Chain=Pierce resistant, etc..). If you knew these vulnerabilities and equipped yourself accordingly, be it on the fly or in preparation, you could afford yourself a nice bonus to your chance of coming out of the fight the victor. Along with that, all the Mobs in the game had the same resistances/vulnerabilities to melee AND magical damage so a bit of research could actually help you out in many instances. Of note, the Archer classes of the different realms were able to equip different types of arrows to take advantage of the same vulnerabilities/resistances as their melee counterparts. (broad-head arrow, pierce arrow, etc...)
Delving even further into the tactics, I know of one class in particular, the Vampiir, that was able to do called shots (chest, arm, head, etc..). If they found that hitting a certain body part on you would proc a nasty little effect on them or heal you, they could change to hitting another body part which was perhaps a bit less beneficial to you.
I think the addition of such features would be nice but considering the added amount of programming behind the scenes, plus added balancing required, and any applicable UI modifications, I think it'd be a feature best left out. Another thing to consider is that the majority of the player base would not even bother with taking advantages of such a system (simplicity typically is a bigger draw) which would render the benefits of that system limited in the terms of overall gameplay and the community as a whole.