It sucks having to learn new units from scratch over and over. Kind of wish there were tooltips when you mouse-over the new units. Took me a long time to figure out that the pollen sprites attacked in an AOE fashion -- I thought they were doing some kind of fog defense. Still no idea what to do with the slimes (besides lose over and over again until I can afford health upgrades).
A "training ground" where you can test units on some escaping captive would also be nice.