I agree with point 1, once I created a summoner things got alot easier. Amazing how unbalanced some classes are. For those who don't know a summoner summons minions instead of towers and uses a completely different DU system(MU instead). Archers, pretty much the only minion worth building outside of special situations, shoot 360 as stated above. This makes them perfect for guarding against dropping spiders or sneaking mobs, etc. With good stats they also rape face as well. They can never shoot quite as a fast as other towers, so super high stats they do not scale quite as well, but they are extremely powerful mid game and are pretty much the only reason I was able to solo my way into nightmare without any shitty gimmicks.
Also as stated above, the EV dude and put "beams" down from point to point. Like a wall, but each some something different. One is a wall, one reflect projectiles(very useful actually), one does dmg and then there is the master beam, a buff. Pretty much amplifies(doubles or more) the range, dmg resistance, and speed of any tower it's on with decent stats. Aka 2 towers and a beam > 3 towers. I never use a tower set that doesn't have a beam on it at this point. Bad news is that if any tower goes down, so does beam and you can get fucked.
I do disagree that insane was ever a challenge though. When I played after it first came out, I was with a few members of this board and we never really had issues except with the damn timer and bosses sometimes. Once you knew where to build to block, you could own everything since it was just hordes of mobs that run straight at you. You had to repair, assist with hero, etc but you never had the unknown happen. The new maps and nightmare added a bit of that unknown which made the game more entertaining because you actually had to "care" again about what you did and where you placed.
You are right about superloot though, even know when I play nightmare it's mostly a joke because my stats are getting insane(2k in tower stats for example, when I left long ago before coming back I had maybe 300-600). At the end of the day I can't quite put my finger on it but I really like this game. I LOVE TD games and this was a really cool play on it to me. Is there a DD2 in the works?