... So, I was curious about why people think it's been nerfed.
Let's see. I don't have my old 4e books to hand here, but I can see a few people saying "No counterspells in 4e" on this forum back in 2010. Some digging finds a couple of specific things that look like homebrew, but it looks fair to say that a generic counterspell that was available to roughly every Arcane caster was not a thing in 4e at all. So, by its mere existence, the 5e version is infinitely more powerful.
Let's see - Pathfinder looks to not have changed much from 3.5e based on the SRDs, so: In 3.5/PF you declare that you are counterspelling a particular target, need to make a Spellcraft check (which, with a range of 15-24, will edge into the "impossible to fail" territory after a few levels) and then cast either a) The same spell the opponent cast, b) A spell that explicitly counters the opponent's spell or c) Dispel Magic, which has a failure chance.
Compare this to 5e: You don't need to pre-declare or use your action. You don't know the spell being cast, but counterspell automatically counters anything lower level than the slot it's in. It has a chance to counter anything higher.
Not knowing the spell is a nerf - you might waste your spell. However, not having to use your action and nominate a single target means you won't ever waste your turn because that target didn't cast a spell. Not having to have the right spell prepared (or resort to Dispel Magic) means you always have a solid chance of countering the spell. Plus, with 5e spell preparation, countering a spell doesn't potentially use up your ability to counterspell for the rest of the day.
My recollection of 1e and 2e are broadly "If you can act before they finish, you can interrupt their casting while it happens", with a few options for it. That's close enough to the 5e system, really. However, 5e does not depend on turn order or initiative bonus, and does not potentially prevent you from casting the spell again later.
Basically, I'm saying that the 5e version is more generally useful and more powerful than the implementations in AD&D (at least, 2e), 3e or 4e. From that viewpoint, the inability to tell what the spell is before you counterspell is for balance, to prevent it from becoming overpowered.