I had fun with it, but I generally agree with your criticism. It's unfortunate, because the action is really well shot, and I would say that for about the first 2/3rds of it, the movie just kicks all kinds of ass and has a great energy to it that works.
It's also very well shot and lit, and looks great throughout. It makes how awful most of the Marvel movies look even more inexcusable, when you see how atmospheric even a throwaway Keanu action flick can look on its small budget. The night-club sequence in particular is action magic in my opinion, and the movie never reaches that high again.
The big problem, like you allude to, is there's no real villain. A movie like this really needs a formidable main bad guy, or bad guy henchman, that is going to be the major threat, and it never materializes in this. John Wick basically coasts through, with no escalation of stakes (I love climactic mano-a-mano fights in the rain, but that doesn't work if you don't have a bad-guy worth fighting). In a lot of ways, this reminds me of almost a 21st century spin on a movie like Commando, but Commando has Bennett as a great final villain, and it also at least has the girl tag-along character to change up the pace a bit throughout the first half.
Still, it gets so much of the details right, and has the right spirit to it, that I'm more bummed that they didn't put in the extra effort to make it great, than anything else. It's a fun one-time watch, but totally insubstantial and forgettable.