@Eve players complaining about the 40 mana on her W.
Dark frenzy has a speed steroid, remove slow, ignoring unit collision, cd reduction per hit, and cd reset on champion take downs. Wow that is a lot of things in a free spell!
This change is not a big deal.
Her current play style is going in with W, spamming her spells, kill the target, and escape with her W. Adding the mana cost will make you play smarter and make you a better player by paying attention to how much you spend. She has runeglaive and a regenerating mana passive, this change wont even affect her that much.

W still blocks half of the skills out there at no cost and
Q gives 3 dashes plus damage plus AoE knockup plus the ability to cancel all her animations. But right, an MS boost on a champ with no gapcloser is OP.