"First to invent" doesn't make much sense when the point of the patent system is to get inventors to publicize their inventions in return for a guaranteed monopoly.
"First to invent" runs counter to the patent system.
"First to invent" runs counter to the patent system.
Little inventors might like to shop an idea first and then file a patent application if they find traction or funding. US law allows for a one year grace period after disclosure. Fail to file within one year, all US patent rights are lost. Outside of the US, any public disclosure before filing bars a patent.
Big Corp has the money to file patent applications early before disclosure.
Now with first-to-file in the US, there still is a one-year grace period in the US, but the first applicant to file is the winner not the first to invent.
Among other things, this is bad for small inventors because bad actors can file patent applications on the small inventors' inventions first. Trolls can and do scour websites to identify inventions that may be worthwhile sniping. In the old days the inventors just need to show that they invented their invention before the troll filed their patent application. Now, you have to prove that the troll literally stole the idea which the USPTO doesn't really look into.
In either case, it's likely that the USPTO isn't going to notice before issuing the patent, but under first-to-file it is way easier (read: cheaper) to show that "X document was published on Y website on Z date" than it is to start doing discovery to compare private lab notebooks or whatever to decide who really invented the thing first.
This only ever affected a minor number of cases anyway, and the change mostly just made litigation costs cheaper and more predictable.
It also seems really great for open source software (where I personally work), since everyone publishes everything they do immediately anyway.
Public disclosure is great for making prior art to block future patents. That's why some big corps white paper just about everything whether they intend to patent it or not.
In particular, the paragraph starting with "Although some of the provisions of AIA 35 U.S.C. 102(a) and (b) are similar to pre-AIA 35 U.S.C. 102(a), (b), and (e), the AIA has introduced a number of important changes with respect to prior art ..."
Edit: And more importantly, Section 2152.01 right below it.
It actually does change the dates for when patent documents are considered prior art (as described in the first paragraph I referenced), so it is easy to get the issues confused.
The effects of first to file on how prior art works are, in my opinion, way more important than the litigation edge condition it eliminated.
But if it expires sooner now, we can enjoy it royalty-free sooner.