However, an unexpected side-effect of the launch of Classic is that I can now judge Retail on it's own merits, since it isn't inhabiting the corpse of something I love. If you think of Retail as a sequel to Classic, it's actually pretty good in its own way. Much the same way that EQ and EQ II are both fun games set in Norrath, or FF VII, VIII and IX are all good JRPGs with a few shared elements, Retail WoW is an interesting take on the same setting as Classic. Admittedly some classes that I enjoy in Classic like the Warlock and Hunter have been completely gutted in Retail. But some classes/ specs that are almost unplayable to me in Classic have also been greatly improved. For example, in Classic I can't stand playing Paladins. They have terrible rotations and are just generally tortuously slow at solo questing. In Retail Paladin's are actually pretty fun. In Retail my Balance Druid is an absolute joy to play (55 so far), and I was barely able to get one up to level 20 in Classic.
Retail and Classic are different games with different core design philosophies. Classic hearkens back to much older MMOs like DAoC and EQ, and includes a lot of details that serve no purpose save to flesh out the world being portrayed. Retail strips many of those out. In many way it plays like a lobby game that happens to be embedded in a MMO, rather than a MMO per se. Things like crafting and a world to explore are there if you want them, but are really an afterthought in the current design. The quickest way to advance a character is to spend most of your time queuing up for dungeons and battlegrounds. However, that's not inherently bad. If you are in the mood for a quick dungeon crawl or light questing where you don't really need to pay attention to anything, Retail is actually pretty good. It also has a somewhat stronger emphasis on narrative than Classic.
Now that you get access to both games for one fee, I think it's actually a good thing that Retail is so different from Classic. It would be similar to getting FFVIII for free when you buy FFVII. One game is a classic that brought a genre that was previously a bit obscure into the limelight, and the other is a controversial game with some highly suspect design choices. However, both of them together is a heck of a lot of entertainment for one price.