"Optional" achievements are okay but kinda lame. I agree that they shouldn't be forced. But they should be uniform across participating games (points given per game, etc) and viewable on a channel. Otherwise it's pretty pointless, since much of the appeal is to see game progress amongst profiles without having to boot up the game.
Maybe I'm underestimating MiiVerse though, but so far I just see that as a glorified GameFAQs board. Maybe that can do kind of the same idea, somewhat. I'm pretty sure most 3rd party ports will put them in, just in game instead of system wide.
One thing that is kinda related and handy that the 3DS has is the play time log. I really like knowing how much time I've put into each title, and it's nice to see that move over to Wii U. Just, apparently it can't track Wii stuff (Wii games, VC) so that's a bit nerfed from my expectations too.
Achievements seem silly to not include. Just another one of those "We didn't think of it so we won't do it" kind of deals that Nintendo is known for. Too bad because they could have added their own spin to it. Like hiring that Smash Bros guy to create an environment where achievement points unlock trophies across the U catalog.
Didn't PS3 patch them in later? Maybe it's not impossible to see something down the line, especially if most 3rd party stuff keep the triggers for them in the code.