Light armor... is about the same. It might be harder to get it to the cap, but it can be done. You lose the reflected damage (you avoid 10% instead - does that bring the total to 90%, or is it still capped at 80%?), and your stamina regens 50% faster, so you can do more power attacks, or sprint more often, etc.
And then there's... cloth. Since the unarmored skill had been removed from this game, players who want to wear mage robes, monk robes, or just want to be fashionable (I'm sure some gorgeous clothing mods will be coming soon...) are forced to rely pretty heavily on alteration, which doesn't do nearly enough. And mages who want to wear dragon masks? Oh... sorry. The Mage Armor perks don't apply to you. (And once you get the master level spell, you might even be grateful if you never bothered to take them. Now that's just dirty!) Of course... that one only last 30 seconds, takes 3 or 4 seconds to cast, and eats up most of your magicka pool, so... hope you brought a staff, because you may not have much left for offensive spells.
This has led mages to just wear heavy armor, forego alteration, and enchant it with -100% magicka cost, so who cares about the regen bonuses from robes anyway? That may be a perfectly valid way to play (let's not debate it anymore, I don't care), but it's not how I personally want to roleplay my mage, and I have seen enough posts from other players to believe that I'm not alone. Besides, let's not forget about the dragonborn who just want to run around dressed like tavern wenches... (hey, to each his/her own!)
I'm not a sit back and whine kinda gal, so I'm working on a mod to rebalance magic skills and make role playing a mage a more enjoyable experience. I've been browsing the forums, but see too much arguing and not enough helpful discussion about how to adjust things fairly. So let's discuss ideas here please, and I'll try to implement some of them into a useful mod.

(For those of you on the console... I feel your pain. I wish we could offer mods as DLC, but Microsoft and Sony seem to frown on that)