Of the games they've revealed...only Tekken 3 isn't available on the PSN store. Assuming they allow M-rated games, it opens a LOT of major titles.
At minimum, I'm guessing the usual suspects like Castlevania: SotN, Metal Gear Solid, Syphon Filter, Twisted Metal, Resident Evil 2, Tomb Raider, probably trying to get Gran Turismo, Crash and Spyro on as well. Probably snag a couple more Square titles (maybe Chrono Cross?). Might be a major boon if they get some rarities like Einhander on it.
I'm a bit mixed. I figured this was coming, but at $5 per title...that's not an appreciable value over using a Vita or PSP, much less any sort of decent emulation box. I'm disappointed they stuck with only 20 titles rather than 25 (y'know, "25 for 25 years"). The decision NOT to use even ONE Dual Shock will limit options for later PSOne titles which took advantage of that feature, unless they plan on sell classic Dual Shocks separately. Which wouldn't surprise me given they aren't even including a dang AC adaptor.
Right now? I'd be FAR more interested in a Dreamcast mini. Aside from Tekken 3 (of the show titles), I can EASILY get and play the other revealed titles on my PSTV. And given the age of the games, 1080p isn't going to do those chunky graphics any favors. We'll see what other titles they show and what featured they might bake into the system.