The premier discard spell and a format all-star. This is one of those cards that when you start playing you tend to think it doesn't do a whole lot and how different it is from other discard spells. However, the effectiveness of Thoughtseize is immeasurable.
I'll start with the easy stuff first, the "downside". For being an unconditional discard spell(well, with the exception of lands), there needs to be a downside and that is losing 2 life. In general, this doesn't really matter, aside from matches against really fast aggro decks and Burn, with some decks, like Death's Shadow, actually benefiting heavily from losing life. However, I'm not here talking about other decks. Back to this deck, the loss of life can be a bit though. There isn't a lot of card draw in black that doesn't require it's controller to lose life(I know things like Village Rites exist but this deck can't run it properly) and normally a Monoblack Infect would run Sign in Blood or Night's Whisper, with the former working better with the mana base. I myself actually used to run four copies of Sign in Blood with a copy of Castle Locthwain and I gotta tell you, that would kill me really, really fast. So, if you, reader, are interested in building this deck and wants to run painful card draw, maybe you should consider something like Inquisition of Kozilek since it hits almost everything you could want. I, however, opted for another route. Instead of not running, I chose to run a new card from M21 to allow me to use Thoughtseize, and that card is Mazemind Tome. I won't be going in depth about the tome right now as it will have its own section. Just wanted to give some foreshadowing and explain that the life loss can actually hurt the deck.
With the easy part out of the way, let's get to the hardest part, its utility. I'm gonna split this into two parts. In one I'll talked about its usefulness as a universal discard. On the other one I'll give my reasons to why I think it's a better option than Inquisition of Kozilek.
When I started this deck, Black had a few problems dealing with specific cards types, mainly Enchantment and Artifact. However, things like Pharika's Libation and card:Fees the Swarm have happened. So, maybe the future has some more of this type of cards. That being said, I'm still gonna focus on the general idea that Black can't deal with specific types of permanents, because that's the reason this deck runs Thoughtseize. For one single mana, it's possible to get perfect information about the opponent's hand and disrupt their game plane on turn one. That's a huge advantage. Against other control decks it can take a removal, counter, boardwipe, win condition or whatever else might be needed to be taken. Against combo decks it can simply remove their main combo pieces or ways to find them. Against aggro, though a little less efficient, it can eliminate a threat that wouldn't be easily dealt with using something like Fatal Push or Collective Brutality. There's probably way more things that could be said about its utility in general but I think I have already gotten the message across.
Thoughtseize versus Inquisition of Kozilek. In a lot of matchups they will do the exact same things, especially if the opposing deck has a very low curve. Their difference comes up with some decks that have threats that, even though have a pretty huge cost, cast those with ease. cough cough TRON cough cough That's when Thoughtseize shows it true potential. I'll try to list a few to have some examples.
These are but a few examples. There's definitely way more than I can remember.
I think that's enough to be said about this one little card. I know that a lot of not every Modern(the format) player knows how good Thoughtseize is in general. I just trying to explain as much as I could to maybe help some people who might consider it for an Infect deck or maybe are really new to the competitive landscape or new to the game in general.
If you want a more in-depth look at the usefulness and power of card:Thoughtseise, the channel Rhystic Studies made an entire video around it. It's definetly worth to take a look.