Clearly defined mechanics and flavor text is a good thing, but does it have to come at the cost of 3rd Edition monster creation? 4th Edition made the process quick and simple, providing clear guidelines on what a monster should do at what level. Of course the "official" books did not always cleave to the formulas, and you could tweak them how you wanted, it was just easier to do so without accidentally wiping out the party and building monsters on the fly.
3rd Edition monsters were basically player characters. You chose the monster's type and subtype, which determined Hit Dice and how various stats like Base Attack Bonus and saving throws scaled. You had to also determine ability scores (which affected everything else), spend skill points, and buy feats. The drawback--aside from all the book keeping--was that if you wanted a monster to challenge a party of a certain level, that it require various degrees of tweaking.
For example fey had a wizard's attack bonus (ie, the worst). So if you want to make a fey critter good with a melee attack, you had to ramp up its Strength and/or Hit Dice. The problem with this is that spells and spell-like abilities--among other things--are derived from Hit Dice, which could make them harder to resist than its Challenge Rating would suggest. That and their own saving throws could make it easy for them to shrug off most effects.
Basically a lot of 3rd Edition monsters where pretty swingy, and building your own could require a lot of tweaking (and bullshitting racial, natural, and circumstance bonuses) to get them where you wanted. Oh, you could also add templates and classes to them, which often resulted in an even bigger mess. I would prefer to retain 4th Edition's simplicity, coupled with monster themes and the degree of flavor we got out of both Monster Vaults.