I'm fine with Legolas being in the Hobbit, since he would have actually been around. I was also fine with Glorfindel being replaced by Arwen.
See, it's not changes to the original text that make me angry; when they're done for good reasons (see: giving Arwen more screentime/development) I have no problem with them. Hitchhiker's Guide was changed in some way for each and every adaptation: this makes sense. Some things work better in a radio play, a tv show, a movie, a book, etc.
It's when they go full Filmamir that it starts to irritate me.
Glorfindel was an elf lord named after the elf lord who killed a Balrog in Gondolin. That's... pretty much the extent of his background. Haldir didn't show up at Helm's deep in the book, either (no elves did). He was put in to show the elves were contributing something, while in the book it was explained (later on) that they were busy fighting off forces from Mirkwood.
The thing about Tolkien is that most of everything is explained in the appendixes, because LotR is from the point of view of the Hobbits except briefly when it's just Aragorn/Gimli/Legolas alone. Nobody told the Hobbits what was going on in the state of the world, they were basically just baggage. So if you want to know why the elves never sent anyone to help at Helm's Deep or Gondor, you have to read the appendixes. Trouble is, most people can't even stand the boring pace of the actual books, much less an index of info dump.