What I really dislike about it is that it makes the concept of Armor Class unintuitive. It's not intuitive to say that the lower th AC, the better, and that the higher the AC, the worse. It should be the other way around, which is what I like about more recent editions.
@m7600 I'm going to start by saying that I totally agree with you, despite the fact that what follows would sound like I disagree with you.
The origins of THAC0 is rather interesting. When they were putting together the combat mechanics for D&D, they needed to have a way to factor in different types of armor. This was before "roleplaying games" were even an established thing so they had not a whole lot to turn to, except wargames. The wargames that had the most viable systems from which to steal were naval combat games. There you would have "first class" armor or "third class" armor where having a lower number class was better (kinda like being in first place as opposed to third place). It made a lot of sense for the players of the time, most of whom were coming from wargames and would have some familiarity with the concept. It stuck and, until WotC changed it, nobody really had a problem with it.
That being said...
"and by add, I mean subtract"
@Fandraxx It's a good video and I get a little nostalgic at times, but I'm kinda glad THAC0 (which I agree is pronounced THAH-co) died. I might actually argue that B/X is the most balanced D&D of all time, but both are definitely more balanced than WotC's products and I would take 2nd ed over 3+ ed. any day of the week. I don't think THAC0 has anything to do with it, though. THAC0 and ABs are just math, and they are (or at least can be) effectively the same computations. However, THAC0 math was not intuitive unless you grew up with it.
I think where WotC lost their balance is not with THAC0, but with the buffet multi-classing and the front-loading character creation. You basically make a D&D character like you would make a deck for Magic. Since most of the mechanics (and often the games themselves, in my experience) revolve around combat, players seem to optimize builds to be exploit advantages with the system. And, I don't mean that judgementally. I did the same thing when I played 3rd edition by creating a Dex fighter who used a fighting chain (the only weapon that had reach and could attack adjacently) and really ran the game on attacks of opportunity. It was slow to win, but I was near untouchable with my trips and disarms. At the time, I don't think they anticipated anyone would build for this. It actually pissed off my DM so much he killed me in the next session with a horde of wererats that were way above our CR.
But, I digress...
I don't think WotC is even trying to be balanced. They're leaning hard into power fantasy on multiple levels. First, it's in the game itself. Characters are meant to be larger than life ass-kicking heroes. It practicallly takes a whole session to build a character in WotC's D&D and you're not meant to die. In classic, you spent a couple minutes rolling your stats and then you had to make it work. If it didn't, you died and started again. Second, it's with the system. An unbalanced system gives players the opportunity to optimize characters and feel like they're smart and "putting one over" on the game. Pandering to consumers this way makes for a happy consumer, and then when you release the products that change the rules and take away those edges, people need to buy the new stuff to stay ahead of the game. Now, I'm probably being judgy. But yeah, when I played WotC's D&D, the whole thing just felt like one big wank compared to classic.