I kind of feel like it's a double-standard. I don't know that it's necessarily or better or worse than the days when players didn't really celebrate anything. They also didn't tend to react very strongly when they were brushed back from the plate. It was just considered part of the job.
I like that there's more celebrating now, but I think it has to be consistent too. Eckersley used to do that fist pump after every final out, regardless of who the hitter was. Throwing the helmet off and group-hugging at home plate after a walkoff hit seems OK to me too. But pause to admire your solo home run that causes your team to be down 6-2 instead of 6-1? It should be thrown at your head next time because that's just unsportsmanlike. This is the kind of thing that umpires should let the players police themselves on.
And the players normally do. Though it took the Yankees too long after David Ortiz twirled in the batters box after hitting a home run. It's hard to police the closers though, in either league. How many at bats do they get a year?