How about a bonus when you first hit a specific wave in each mode. Say, a 50 gem bonus for hitting Wave 500 in Normal, a 250 gem bonus for hitting Wave 500 in Hard, etc.?
Tutorial Green Hand won't go away after I upgraded my Twig Snapper, and locks up the game, allowing me only to click on Twig Snapper. Terrible bug, ruined the game.
The gameplay control is just too unfriendly. Selecting and moving units is almost a "click-and-pray" exercise. What is a pretty decent concept becomes exceedingly frustrating. Why not allow selection of multiple units?