Sona's utility needs some serious power boost

For a champion who is basically all about the team utility, Sona's utility leaves A LOT to be desired. Her mana costs are painfully high, her range is a joke for a champion as fragile as she is, and she is slow as dirt, and she is so reliant of AP that you can't even get up to par till 15-20 mins into the game. Plus all of these don't even touch on the fact that her utility is really feast of famine. While i will readily admit her utility is probably #1 in the game if you can be tagging your whole team with each cast, as soon as you start missing even one ally those skills start seeing huge diminishing returns. "only managed to tag two allies? well it's not garbage but it's certainly isn't award winning.... only tagged one ally? why did you even bother casting the spell!!"

Her movement speed is probably the most painful problem to deal with. given that she lacks a stun or escape tool (outside flash). Having some of the lowest base movement speed in the game just makes her a painfully easy target for even the most basic champions to catch. I understood this issue back when her aura's were always on and had a 1000 range, people should feel good if they catch her. But now you have to be near the front lines to be helping your team, so being super slow to boot just doesn't work anymore. For those who mention her E speed boost, this would be wonderful but it has a HUGE cooldown and until you get a couple points into the skill max your CDR and have a fair chunk of AP it just doesn't give enough of a boost for long enough to really provide a meaningful use of the skill. And this is the big issue even at max CDR the skill is still on a 7.2 second cooldown, given that basically every other character has some form of gap closer, long range slow, and/or higher base movement speed there is basically no way to escape an enemy who is chasing you, or run down a target who is fleeing from you. Two things to keep in mind are that the movement speed bonus starts at 3 seconds, you can get an additional half a second per ally tagged but this still means that even if you tag all your allies you will have just over 2 sec where the aura is not effecting you. The other key thing to remember is that the aura decays over time meaning that while the 13-23% movement speed bonus sounds amazing you often will not get the full bonus for more then a brief moment meaning that again you will not be able to sustain the speed you need to be hyper mobile around the map.

Let's now move to her W, heal/shield. This skill is very cost prohibitive in the early game and in the late game it often doesn't provide enough of a bonus to really swing fights, often feeling like a win more ability. At the start of the game the spell costs a whooping 80 mana to cast, and heals for a pitiful 30 per target, with a half % boost per 1% missing health, meaning that if you heal a target who is at 1% health you do 45 points of health. Yes i am aware that the spell heals two targets mean that for the cost you are getting a base heal amount of 60 with a possible max heal of 90. Even with this factored in the mana cost to healing done is still painfully lacking. To give a counter example, please look at Taric (the only other champion with a split heal), his mana cost starts at 60 and heals both targets for 60 health. Meaning that for less mana this champion can out heal you by 33-50%. If we look to the late game this skill often stops being about the heal and becomes more about it's secondary effect (the shield). Here we run into a different kind of issue. That being that her aura range is painfully low forcing her to be in the middle of her team, who all need to be very bunched up, to apply the shield properly. The shield also has it's own set of problems, those being that it doesn't last very long 1.5 seconds so if not used very selectively it can fall off an ally before it absorbs any damage. The other big flaw is that the shields on their own aren't all that impressive assuming maxed W ranks and a maxed ultimate your skill provides a 150 +20% AP shield to your allies. This will usually mean blocking 1/4-1 of an attack (auto or skill), not all that impressive if looked at individually. This skill much like her E would probably be amazing if you could reasonably spam them, but since this skill has a 10 second base cooldown, meaning a 6 second cooldown at max CDR, you often will be unable to cast the spell more then once or twice in a fight. The skill might also be saved if Sona's aura range was large enough that she could skirt the edges of the fight for better positioning. However since all her "auras" have a super small radius, you will often have to be in the middle of your group in easy engage range of your opponents (and since she is so fragile even mild focus will kill you quickly).

TL:DR

Sona is to slow and lacks engage/disengage Her E doesn't compensate enough and isn't off cooldown enough to make up for it. Sona's heal is too weak to matter in a fight, and costs to much to allow for healing after skirmishes.
Her shield is unimpressive when looked at what it offers to each individual ally. Her Utility cooldowns are so high you can't use the skills often enough in a fight to have much effect on the fight (even with max CDR) Her aura ranges are to short to allow her to be on the edge of combat instead of in the middle She is too fragile/ easily focued to have to be in the center of combat