I've always liked how the longer Bruce is Batman, the more it consumes him. In B:TAS, he's more like a tough guy with a big heart. He'll crack a joke and a smile but won't hesitate to punch a criminal in the face. Dude even plays around with enemies like The Joker. Take this piece of dialog from "The Last Laugh":
*Joker is hanging over a vat of chemicals*
Joker: Are you going to let me fall?
Batman:
Joker: Batman!!!!
We also see him trying to reason with some of his more tragic villains like Two Face. A Bruce Wayne, we see him still joking around at some points and having a good time. By the time we get to the TNBA - he's more dark, seasoned, and reclusive. Take for example the episode "Sins of the Father". Tim Drake comes down and asks Batman and Batgirl if his father is dead. Barbara tries to sugarcoat but then Bruce out of nowhere says "Yes.
" And he's done reasoning with Harvey and some of his other villains too. It's now like "No more reasoning.
I'm going to break your face in two and send you back to Arkham in a body cast if I have to." He's still smiling, but less. It's almost a rarity. Then, by the time we get to JL/JLU - Batman has fully consumed Bruce. He's seen some things over the last decade as Batman. The Batman/Bruce you saw in B:TAS is gone. I believe the only time we see him give a little smirk is in "Injustice for All" (when he knocks out The Joker) ,"This Little Piggy" (when Wonder Woman walks off humming "I'm Blue"), "Ultimen" and "Destroyer". That's all that is left of the B:TAS Batman. But this was when he was at his best IMO.
Then by Batman Beyond?
He's old, bitter, broken and alone. Crime is still going on and he can no longer protect Gotham due to being old. He lost control of his parent's company and it's now a front for many illegal operations (including murder, conspiracy, etc.). His relationships with Batgirl, Selina, Diana, Talia, etc. never panned out. He probably constantly thinks about his last fight with The Joker in the broken down Arkham. All of his allies (Jim, Alfred, Lucius, Bullock, etc.) are dead and he's cut off all ties with the Justice League and everyone in the Bat family (once again due to the flashback events of ROTJ). With the exception of his dog, Ace, he's alone and has no hope...until Terry arrives. And it's like a fresh breath air. He can train this kid and carry on with his mission even though he can't be Batman anymore. He's still stern, cold, and calculating while he teaches Terry the ropes but he's not above making a wise crack and smirking about it afterward.
Bruce: Oh and Terry there was one other thing...
Terry: *in the middle of battle getting lasers shot at him* Yeah?
Bruce: She wants you to pick up some milk on the way home.
Terry: MILK?!?!
He's even that way in "Epilogue" (by this time in his 90's). In fact, Terry is now more like Bruce (with a few differences like wanting to settle down) and Bruce is now like Alfred:
Bruce: Kent called. Nothing apocalyptic. Just needs your help on a case he's working. Says you can meet him at the Metro Tower.
Terry: Right. Better suit.
Bruce: You should eat something first to keep up your strength.
Terry: When I get back!
Bruce: You're a stubborn piece of work you know that?
Terry: Just like my old man.
Like father, like son.