A little vibrato is all right, a tad of glissando isn't too bad, and even a little portamento here and there never hurt anybody. Of course some may say that it's all a matter of taste, of opinion, or even of fashion. But don't you believe them.
Take, for example, Itzhak Perlman's mid-'70s Decca recording of Beethoven's Kreutzer and Spring sonatas with pianist Vladimir Ashkenazy. Nobody would say that Perlman doesn't have the chops to nail the notes; anybody can tell he can play to make the fur fly. But even in a showoff piece like the Kreutzer, the player is not the point; the music is the point. And just listen to the way Perlman squeezes the notes with vibrato: doesn't it sound like he's squeezing grapes, that is to say, trying to make them whine? Listen to the way he slides up into the notes with glissando: doesn't it sound like he's not altogether sure what note he wants to hit so he hits them all? And listen to the way he glides between the notes with portamento: doesn't it sound like he's not completely convinced that the notes themselves are enough to make his case so he adds the notes in-between to make sure?
Maybe to some people that's alright. But this is Beethoven we're talking about here, not a late Romantic like Brahms or Bruch or Tchaikovsky, and Beethoven's a clear-eyed, lean-muscled, strong-willed composer, not a stiff-necked, narrow-chested, thick-waisted composer and to cover his music in a heavy coating of vibrato, glissando, and portamento is as wrong as playing Bach on the kazoo.
Vladimir Ashkenazy's habit of putting the pedal to the floor blurs the harmonies, blunts the phrasing, and smears a gooey sonic haze over the forms. And, for better or for worse ,Decca's rich, deep, and full sound makes it all only too audible.