I've actually made a list of reasons:

-Terry McGinnis. Say that out loud. Terry. Mc. Ginnis. What a generic teenager name. It sounds like the name of a guest character from The OC or something. Just a completely lame and forgettable name. I can't take it seriously.

-The show indicated that Batman and Batgirl had a romance. WTF, that is fucking retarded. There's a big age gap there. Can you say ewwwww? Plus, she dated Dick Grayson. Who thought up this horrid idea? Batman already had plenty of love interests, like Catwoman and Talia and Zatanna and later, Wonder Woman in JLU. Why give him another fucking lay? This was garbage.

-Batman TAS looked like it was the 1940s-1950s, with the old fashioned cars and blimps and art deco buildings. Batman was 30ish then, and now in BB he's 85. So if it's 55 years later, that would mean it's about 2000 or so. So why are there hoverbikes and laser guns and shit? Gotham is a futuristic neon mega metropolis that looks completely unrecognizable. This makes no sense. It's too futuristic. I do like future stories with Batman, like DKR and Kingdom Come. Those are way way better then what they did with Batman Beyond. You don't see ridiculous garbage like hovercars everywhere.

-The core concept of Batman being replaced is horrible. Bruce Wayne is why I like Batman. Honestly, I don't even like Robin or Batgirl or the idea of the Bat-family. To me, Batman by himself is the most enjoyable facet of the legend. Christopher Nolan's two films do not contain a trace of the Bat-Family. Most of the best episodes of Batman TAS have Batman by himself. Heart of Ice, Beware of the Gray Ghost, Mad as a Hatter, Feat of Clay, Two-Face, Day of the Samurai, His Silicon Soul, Sideshow, Catwalk, Perchance to Dream, Zatanna. There are a few exceptions, like Baby-Doll and House and Garden.

So to move outside of the Bat-family and have the persona adopted by some nobody punk teenager is insulting. It's like a Poochie the dog moment. The entire show is about the Batman identity moving beyond Bruce Wayne, and I have no interest in that idea. Bruce Wayne is Batman to me.

-The entire show being about a young teenage newcomer learning the ropes and being a bit of a wise cracker is a lot like Spider-Man, not Batman. Guess what, I'm not a fan of Spider-Man. So I definitely don't like the same formula done for Batman. Batman and Spider-Man are not similar, and forcing a Batman to be like Spider-Man sucks.

-Terry McGinnis doesn't do shit. His suit does all the work. The armor enhances his strength, and provides him with flight. Not just gliding like in the Nolan films, but actual powered flight with rocket jets. This is more of an Iron Man shtick, not Batman. Iron Man's fine, but I want something different with Batman. The attraction of Batman is that he relies on his own skills, training to the very pinnacle of human achievement. Not just being some average teenager relying solely on technology to save his ass.

-The rogues gallery probably didn't match the original Batman gallery. I mean, Inque's a pretty good villain, I'll give you that. And the romance with that girl in the Royal Flush gang seemed interesting. But cmon... you aren't gonna be able to match Batman's villains.

-And I never watched this episode, but apparently there was one where we all found out the amazing plot twist that Terry McGinnis was actually Bruce Wayne's biological son all along. Sorry, but this is just... stupidest fucking revelation ever. It makes no sense. It's the worst thing since sliced bread. It's worse then midichlorians. It's worse then M Night Fucking Shyamalan's twist endings. It makes no sense. It's retarded. Talk about the biggest fucking coincidence in the history of the universe. And the Phantasm from the TAS film was also somehow involved, this old lady in that ridiculous Phantasm outfit. WTF. Whoever thought this crap up should go kill himself.
 
Fuck Batman Beyond.

" The animated series wasn't actually set in the 1950's was it?  It may have had that look but technology wise, i remember it being pretty modern.  "

The style of the show you're referring to is more 1940s than 1950s, but it's just that—the style.  Style doesn't indicate time frame, or else tourists to Washington D.C. might wonder if they'd stepped through a portal to the 1800s.  You have to first understand that Batman TAS is not set in "our" world, but in the world of Batman, and therefore is not constrained to look like our world—the deco period of Gotham may have come just prior to a leap forward in architectural aesthetics and scientific achievements.  Often when advances happen, they happen quickly and profoundly.  Only ten years ago I still owned a VCR and a 20" not-even-flat-screen t.v.  None of my music was digital.  I owned a phone that was not a cell phone, and it was attached to an answering machine.  Travel to 2001, jump forward in time to 2011, and you'd wonder how things got so different so fast.
 
On the point of Batgirl & Batman having a relationship, it's actually MORE normal for an older man to have a relationship with a younger woman than it is for the man to be the younger party.  And even if Batman were in his 50s when he had the liaison with Batgirl, she would have also aged some by then as well.  Yes she seemed young in BTAS, but remember that Barbara Gordon has at least one college degree (in Library Science) and spent time as a Congresswoman (though perhaps not in the BTAS universe).  Nonetheless, if she were 20 when Batman was 35, then when he's 55 she'd be 40, which is entirely viable, as she would by then be well into her prime of adulthood.  And if he was 75, she would be 60, not still a teenager or a twentysomething.   I'm not entirely certain that Bruce "adopted" Dick, either—and if you say that he did adopt him in the comics, then that makes Babs even older, as in the comics, and much closer to Bruce's age.  Anyway, this is why Dick's last name remains "Grayson"; but certainly if Babs and Bruce had an implied relationship, it wouldn't have been creepy, but responsible and adult.
 
The fact that you can't take the name Terry McGinnis seriously is not an argument that it isn't a good name—I couldn't name many comic book characters named Terry, so it's definitely not generic.  Nor is it forgettable.  You obviously remembered it well enough to write about it.  But really what I'm wondering is, if you're not interested in moving beyond Batman's identity then why bother writing an article about a show whose premise is to move beyond Batman's identity?  That's like saying I'm allergic to peanuts, then getting pissed off when I have a sinus flare-up after eating a Reese's Cup.  "Fuck peanuts," I'd say, hoping someone would see the logic in how I shouldn't put peanut butter on a slice whole wheat bread—my rationale being that grape jelly was already on the other slice.

I disagree with a lot of your sentiment here, and agree with some of it. Most notably that I have a hard time equating Terry with Batman, I usually end up saying things like "and then Batman told Terry he sucked and to grow up." Most of it is opinion, which you're entitled to, so I won't try to argue that.

But I would like to say that there was an episode that dealt with Terry losing the suit to a computer program, and he said he often asked to himself whether it was the suit that was Batman or the person inside. He then went and defeated the suit with his own skills, however few they may be ^^;

@Delta_Ass: Batman Beyond is a different universe from the one in Batman: The Animated Series.

Anything could have happened. Dick Grayson may have never dated Barbara Gordon.