Are all of the creatures, other than those that've been removed, still the same? If I'm going to be playing a game that's made a decade (plus?) later, I need more of a reason than candy graphics. Improved gameplay mechanics are nice, but...

I want something more than just that. I played the HoMMs for the world more than the strategy. And I'd want there to be something new and different about that world.
No, all the factions have been completely overhauled, many new creatures and every creature now has multiple abilities.
The battle screen is much more tactical now, you can just outright attack, or you can be more crafty using creature's abilities to change things.
For example, the Haven faction has a 'priestess' type unit, she has an ability to resurrect some dead.....she is pretty useful in a battle for attack or defence, but vital if you want your stacks to live longer. Creatures are more then just meat chess pieces now.
The campaign is rather interesting, lots of story stuff there if you are interested in the world....though it's all about unisoft's re-vamp world (if you didnt know, when ubisoft created 5 they chucked out the previous lore from the series and started again).