While recogning and righteous fury were aways pretty scary, you couldn't keep it up cause of the absurd mana costs, you were forced to buy her mana early game to sustain her abilities in lane, not to mention the cooldowns were pretty annoying. Compared to other old champions, her early game was considerably bad. In order to reach endgame facerape potential you had to farm alot and go straight up offensive and rely on your heal and ultimate for sustain, which meant that a non optimal early game would ruin her through the whole thing. Since she is (was?) a snowball carry, if you got a good early game with her you were pretty much set, if you got countered you'd be useless until you reedemed yourself with CS. She's like Jax, a mess of an AD/AP hybrid which either works really well or fails, except she's worse then Jax because he is probably the first tanky DPS in the game and gets progressively harder to kill even if he doesn't build a single defensive item. Gonna really miss her old passive though, her new one better be really good.
What makes old champions seem bad is not that they're actualy bad, but because they were designed with another metagame in mind, one in which you spent a long time farming then became a monster in the late game. When champions started being more consistent, the older champions started getting more unpopular since they could be countered by champions who are alot stronger in the early game, and some of these didn't even get weaker as the time went by (remember pre-nerf xin zhao?).
Now that she supposedly has a more consistent and sustainable early game, she'll be alot more viable against the current tanky DPS metagame. Hell, I'm happy enough they changed her model and animations, Kayle's were probably some of my least favorite ones, with only anivia having worse animations in my opinion. Plus, her old model looked sort of unpolished. Too bad she still a lame voice :C
Now I just hope that Sivir, Morgana and Mundo get some love sometime soon. Morgana also suffers from abyssmal cooldowns, Sivir is outdated as hell and Mundo just feels out of place after his nerfs.