The darkside as portrayed in the OT was about power, as you could feel it as it takes over your body. Just watch Luke as he is tempted by The Emperor in ROTJ, it is overcoming him, and he is becoming this powerful mother f-er, and that is the true trap of the darkside, it is like a drug, the high you get from it is tough to give up.
Now fast forward to the PT, the darkside has a totally different context now, as it isn't about Anakin becoming more powerful, it is about him being duped or lied to. Anakin never becomes more powerful once he takes the pledge to be the Emperor's apprentice, as it never overcomes him, he is just tricked into thinking it will. For the rest of ROTS, he is no more powerful then he was before The Emperor called him Darth Vader, cause he essentially loses to Kenobi in a fight, as you see nothing in that fight to prove that the darkside has made him oh so powerful.
ROTJ and ESB are just about temptation and feeling the darkside, and knowing once you go down that road, it will overcome you and you will be much more powerful, hence Luke kicks Vaders ass in ROTJ. The PT darkside is just about a trick by Palpatine, and that is why it comes off so lame, cause it doesn't match up with 4-6 when you try to watch them in the same context of one movie saga.