When the developers sit down and decide to work on something, they usually go "What will have the greatest benefit to the number of accounts that'll be subscribed?" In other words, they have to choose stuff that takes the least amount of time for the greatest number of subscriptions. Now, of course, this is an idealistic account. In reality, the developers sit down and pull things from a hat, and some jerk replaced half the pieces of paper with stupid and pointless things, like /death fix...
Anyways... I'd say that if the developers simply added monks and druids and gnomes and artificers... etc. It'd have a far greater impact on accounts than adding 11 more high end dungeons or putting in 4 enhancements that only a portion of 10% of the players will ever take. Pretty much all players, new and vet alike would like tinkering around with the new stuff. Every time you add a new race or class, ALL of the content that has come before becomes relevant again. A few more dungeons? Most players will be tired of them in no time at all. Look at WoW:TBC. Most hardcore players got through the enormity of that content in 10 days...