OK. These are all the Keeper mode; I haven't tried Adventurer.
Great:
Random worldgen and having varying resources. Makes life much more interesting.
Controllable research tree, including found-techs. Nice not having to simply wait for the warlocks et al.
Controllable minion-recruitment: make the army you want, more or less.
Manually having a "dispatch creature to location" function. No substitute for a good training-room replacement, but nice for starters.
(Simply having the game.)
OK:
Minion variety. I like having golems, undead, etc. Beasts are pretty much wasted research ATM because they're not orderable IMO though. Tough to tell the exact differences between the various types, though (is a mummy more effective than a stone golem, for example?)
Having most all constructions available at start
Prisoners as labor force (though I'd imagine they'd try to sabotage/escape?)
Varying enemy races, w00t
Critters automatically grab gear as available
Not sure what the Black Market stuff does, and would be nice to be able to sell off or otherwise process surplus enemy gear
Archery's Cool, but tough to tell who can/can't use it and whether it's worthwhile
Iron's a little too necessary as compared to stone or wood
Not so good:
Hostiles don't repopulate, requiring one to eventually send strike teams (which don't seem able to take prisoners!)
Prisoner-taking doesn't feel reliable
"Boss" hostiles are practically invulnerable (rarely if ever hit them, then they potion it away, making the fight between my Legendary Humanoid and the Duke, rather than a climactic struggle among the heaps of our fallen troops, a tiresome game of "who runs out of potions first?" (Bet the Duke to win that one.)
Imps, etc can't pass boulder traps, breaking "rat run" mechanics; in some cases they can't open doors either
Mana gain is way too slow in the endgame; Vampire Lords' "study" ability apparently doesn't do anything?
Critters aren't particularly good about making sure the artifact gear gets used. Whack one Lord, use its weapon on the next!
Can't name Keeper or critters. I'd definitely not want to have some backer's name on my Keeper.
The Keeper xemself;
+Nifty having a physical presence, cf. Evil Genius
+The idea of being able to support my troops personally is promising
-Only reliable way to gain mana (= troops and techs) is to leave the Keeper parked in the library though :-(
-Keeper spells are poorly defined and no direct-damage available until/unless master sorcery
-Rear-echelon Keeper nevertheless takes most of the potions, rather than letting the troops take 'em to the front
-Keeper can't be shifted to the Lab to make potions xemself (support vamps, etc) or otherwise directly intervene.