The Red Wedding was bigger, but Tywin's death was huge. Saying "(it) had little to no effect" is fucking wrong, and you should feel bad about yourself, Vex. Just the fact that it brought Cersei into power had a huge impact on everything.
Cersei was already in power before Tywin rolled in (though Tyrion did his best to rout her); he hadn't, as of yet, managed to replace the people on her high council. He was still trying to reconcile peace with Highgarden; the most he did during his "reign" was squelch the rebellion in the North, with help from the Boltons/Freys and that whore of a mother of Jeyne's.
What, besides the RW, has Tywin done that his death undid everything? He was consolidating power and working to get Joff away from his mother (he didn't, but luckily for him Joff died) then he tried to get Tommen away from his mother (he didn't), he tried to get Cersei remarried and out of King's Landing (he didn't), he tried to reconcile with Dorne by letting the Viper get some small amount of vengeance and then "killing" the mountain to appease them (he didn't, and when the mountain died of the poison he sent the skull to Dorne but Doran was not mollified.)
So, really...what did Tywin do, that was so huge that his death left a gaping hole in the story? Don't get me wrong, I love me some Tywin because it seemed like his influence might turn shit around, but other than a hopeful "what could he have done if he had lived" he is another flash in the pan. If we're saying his death is huge because he was actually good for the realm, then yea I'll agree. If we're saying his death was huge because there are probably tons of his plots left to unravel, then I'll agree.
As it stands right now, the majority of what he worked for was for naught, there's no one left to avenge him, as Jamie found his honor, Tyrion was the one who killed him and Cersei is too busy being retarded.
The RW was gigantic, because it threw a shit ton of things into motion. People changed sides, wars started and ended, marriages were made, new alliances formed etc. That was Tywin, but it didn't require his death. What did Tywin's death do other than cut him off from making the realm stable? He wasn't sending men to the wall, the attempts at "peace" were being ignored because he is not well loved...Tywin had played his part.
When Tywin died, can you even remember what happened? What effect did it have *on anyone* beyond, "what might have been"?
If you actually think, there's nothing that his death caused, created or stopped that cannot be solved another way. The realm will have stability, others are working towards it, and that was Tywin's claim to fame.
Admittedly, Tywin was a person to be feared, the name was enough to cause people to quake, so taking him out of the picture might have changed some people's opinions and their actions. But bigger than the red wedding? Mattering more than a few POV chapters after his death? There is no one who wants to seek vengeance for him, no one to pick up his mantle. His death did nothing but stop the 4 chapters of "almost good realm ruling" before Cersei took over again, like she has since near the beginning.
A "what if" just doesn't do it for me, sorry. Considering he was also fostering a good bit of hatred in the north prior to Robb Stark's death, hoping that everyone would fight amongst each other and not be able to defend the realm, then yea, you know. It's better he's out of the picture, because we're going to need some assholes who realize what the fuck is happening in the north is about to steamroll all over their stupid plots.
Also, remember what Varys and Doran both said - they had plans to take Tywin out of the picture already, a good deal of plots required him to die. His death simply came earlier. It changed nothing other than slightly speeding shit along and taking a boss of a character out of the book.