Not much, really.
There are four audiences: Congressional Republicans, Congressional Democrats, the public, and the press.
Congressional Republicans basically don't give a shit what he says. They are divided over Obamacare. The hardliners want a repeal with only a symbolic replacement, the few remaining moderates, like Collins, want the replacement to be at least vaguely comparable in scope and the tacticians basically don't want to go through with repeal because they are afraid that if they do it, they will face electoral penalties the way Democrats did for passing the thing back in 2010, as I have explained before. Nothing Trump says will resolve these differences. And if he starts demanding a trade war, well...
Congressional Democrats are just going to seethe. Nothing he says to them matters.
The public is similarly polarized. You can check in on Trump's approval ratings over at Gallup any time you want, although I'm sure Trump would call it "fake news." Most of the public won't watch. The only people who will tune in are the politically active. That means partisans. Nobody will be moved.
Then, there's the media, who have become separate actors because Trump has made them so to an extent that no previous political figure has. He will get good coverage from Fox no matter what he says and bad coverage from MSNBC no matter what he says. Now, part of that is that the ideological content of his speech will be conservative, having been written by other people who actually know what conservatism is. There is no way that a conservative network will bash a conservative speech, even if badly given, and there is no way a liberal network will praise it.
I don't think I've missed anyone, and the basic point is that everyone's reaction is basically predetermined.
Of course, though, presidential addresses can come with moments like this:
Congressional Democrats have about as much respect for Trump as Rep. Joe Wilson had for Obama. Wilson fake-apologized for doing this, and then raised a shit-load of money off of it. Ultimately, Wilson's stunt didn't amount to much, except that I still reference it, but I wonder what the Democrats have in store for Trump.
This is all pointless theater. Go read a book instead. I have to watch this shit, but you don't need to subject yourself to it.