They're really pushing to force Irelia top instead of mid.
They removed the magic damage resistance from her W and made her need 5 stacks, but she is significantly stronger at max stacks than she was at four previously. She even has more attack speed at 4 stacks compared to 4 stacks before.
She's also got more health growth, more base armor, and her W is stronger against physical damage.
It's a shift in power, not a total nerf. They're gearing her more towards sustained damage, long trades, and teamfights, rather than short trades and burst damage.