Let's see...
A combination of new monsters and old conversions. The Fiend Folio probably had the nicest balance for me (although I didn't realize it at the time, not being well versed in Planescape obscurities). Check.
If they are to do all new monsters, which I can't really blame them for, make them exciting! Give them cool new mechanics! Make them visually interesting! To take the MMIV as an example, bloodhulks, wizened elders, justice archons... all blah. They're phantom fungus monsters. "We need a CR 7 celestial based on a mini! Stat!" Whereas, say... the balhannoth, zern and vitreous drinker all had memorable appearances and neat abilities.
Less outsiders? Please. Or, if we must get them, less outright demons. More devils! More yugoloths! More monsters from relatively underpopulated areas! (That's the reason the windrazors appealed to me, I think - they're from the critically empty Pandemonium). WoTC has always put out demons at a great rate, and I hope Fiendish Codex II goes a long way to reversing this trend.
A reasonable level of Tiamat spawn. I can accept that they're going to be a common thread from now on - their presence in the Dragons of Faerun ToC confirms this. But Dragons of Faerun does it well. There's two of them. I can accept two of them in a MMV. Another 12? No thank you!
Creativity in presentation of classed humanoids. Really, I'd rather not have pages upon pages of classed examples at all. But if they are included, please make them flavorful and interesting as opposed to bare-bones and obvious. In other words, less drow arcane guards and Lolth's Stings and more orc plaguespeakers and lizardfolk wasp cavalry.
No maps! No maps no maps no maps! Even if they're good ones! I don't really have a logical reason for this, just pure unbridled prejudice against maps in my monster books. More maps = less monsters!
Tweaking the statblock. Again. Or, rather, putting it back a step or two. I admit to distrusting the new format as opposed to the "classic" 3.5 statblock. The statblock used by Dungeon for about a year or so has three little things that make it miles better than the MMIV's. Lines in the block for Treasure, Environment and Alignment. We don't need a paragraph explaining why the horrible soul-drinking thing from Hades is neutral evil! And we certainly don't need a paragraph on how it keeps 600 gp when a simple, concise "standard" will do. I just did some writing in the MMIV style, and let me tell you - it sucks when you have a word limit. It eats up the wordcount like it was popcorn.
Well, that's my story and I'm sticking to it.
Demiurge out.