One of the things I've had to force myself to do is to break my one-class specific mindset as I level alts. I played a lot or warriors as I went from server to server to play with RL friends, raid with friends from my first guild, and then join a new guild in the faction I had yet to really play. As a result, for quite a long time I saw the game entirely through a warrior's eyes: what was good for warriors was good for the game, what was bad for warriors was bad for the game. Admittedly, there are a lot of warriors. But by far the vast majority of players use mana and not rage, as an example of an aspect of the game I was unfamiliar. My first abortive character in the game was a paladin that I took to level 33 before abandoning him, and as a result I seem to have a mental block about paladins: every time I start to try and level one I stall out. (My BE pally seems stalled at 48, for example, I just never seem to play him.)
It wasn't until I started my orc shaman that I started to understand how MP/5 helps, what good Int is as a stat, the difference between caster stats and healer stats, why some weapons have equivalent damage and healing while others have a lot more healing than damage (or no damage, before the most recent patch) - I've now played two shamans to 70, but that doesn't mean I really understand how the game works for, say, mages or rogues (to name two classes I absolutely cannot play worth beans) much less druids or warlocks.