Poor gameplay mechanics and representation of vampires in Skyrim aside, I like the approach TES has on vampires. When you feed you don't really become weak, you become more "normal". Therefore you loose your vampire characteristics as long as you feed. You become an addict for blood and when you don't feed you are going through a blood feeding withdrawal. Making you more monster-like, your powers of illusion grow, you are "stronger", more vicious, more dangerous. Maybe because psychologically you are a mess, this blood addiction is making you go into a frenzy and the powers growing stronger is a representation of that. Anything that can help you get your fix faster. You are a junkie.
Gameplay wise I think it should be reflected in your character. You look like a normal nord (for example) when you feed and by the second stage maybe your eyes go orange, third stage your face begins to transform into this monster thing and stage four, automatic vampire lord. You burn in the sun, you need to hide in some cave 'till it's dark and then you prey on some innocent victim.
Sort of like Michael Morbius:
Feed or normal: http://images3.wikia.nocookie.net/__cb20120212092359/marveldatabase/images/6/61/Michael_Morbius_%28Earth-92131%29_001.jpg
Stage 2 and 3: http://images1.wikia.nocookie.net/__cb20080805220936/marvelanimated/images/2/26/Morbius.jpg
Stage 4: http://images2.wikia.nocookie.net/__cb20120212092447/marveldatabase/images/5/57/Michael_Morbius_%28Earth-92131%29_005.jpg