Drysc made a rare appearance over on the Rogue forums this week to explain the reasoning behind the haste rating changes scheduled for patch 2.2. Melee haste has been reduced in effectiveness, while spell haste has been increased. I'll try to answer in my own words, I know this has been a large complaint from the rogue community and I've only just recently gained information about the reasoning for the change myself.
Our baseline melee stat is Attack Power, it's the stat that we use as a benchmark in which we compare all other melee stats. Haste was at about a 1:1 in relation to AP at base levels, and it scales a whole lot better. So in large quantities haste was easily overcoming AP, and trouncing all other melee stats. While it's not bad for it to be a strong stat, we don't want it to become THE stat over AP. In fewer words it's a quintessential nerf: it was too good.
In the caster case, it was difficult for us to judge just how haste would affect casts as it varies so dramatically from player to player, class to class, and even spec to spec. Originally when it was nerfed we were sort of anticipating it being required, but we're just not seeing the large use of sustained cast-time oriented spells we thought we would, and it's being brought back up to compensate.
