Just a few comments:
1) graphics doesn't make a game. There are games I'd even dare to all ugly that are fun to play - and I've seen a truckload of polished garbage
2) yea, balance issues are known and modders might address them...
3) How do you imagine "multiple winning strategies"? That some maps would be easiest to win using just one class? (BTW, there obviously are multiple winning strategies - mine doesn't have Ice mages in it ).
I see the depth of the game in variety of character builds within class.
4) This applies to all games and there's really no reason to push for a solution - for one simple reason - the ability to grind when the game becomes too difficult for you allows even less skilled players to enjoy the game. If you overgrind, that's your fault, not the game's fault. Perhaps, however, what the game could do is, based on betatest player experiences, annotate the maps for something like "recommended level", so that players would know that attempting this map before the army core is at level X would be very challenging.
5) I wonder how many people actually listen to game music (long-term). In most cases, I appreciate the soundtrack but after a few hours/days of playing I switch it off anyway even if it's very good, and play my own music...
6) I'm not a big fan of variety just for the sake of it. I don't mind more unit types as long as they have a significantly different "feel" for the gameplay. So, how should the trolls behave to not be just knights without armor?
7) More goals is certainly fun, i.e. extra spawn points like the ones in Gemcraft Labyrinth. I don't like the idea of unit spells - either they'd be automatic, and then it's just another unit skill, or it would become a bit too micromanagement-oriented game.
8) Since the leader doesn't fight and essentially is just the exit point to protect, I don't think it should be movable. More spells? Why not, but again, come up with interesting ideas. Now, with the skill cap 15, there's still more than enough spells
9) Erm, there are actually a few more main groups of bad guys, and I'd say that quite matching the TD standard set of normal, fast, tough, armored, spawning, crowded... The only "classic" missing type is flying enemies, but we have ranged attackers instead, plus the stealth darkies - and NG makes it far more interesting with a variety of extra properties. I would definitely NOT call it a letdown in variety.
There is a known balance issue, I've already reported that as there is a big jump in difficulty that should be addressed.