A practice mode would be good. It can be frustrating because most games you play is on a team in which players don't understand even the fundamentals of TD as well as playing against a team just as clueless.
What's interesting is that, in spleef for example, only few of us sneaked on the snow melt layer. Before you know it, more and more people started to do it. Now the majority of people know about that little tactic and almost everyone sneaks.
This suggests that people on here are capable of observing experienced players and learning from them. Yet, this rarely happens in TD. Instead, when you try to instruct someone to, say, not spam mages or other effect towers, they will tell you to shut up or ignore you or tell you that you are wrong. People are eager to observe and try out new spleef tactics, but unwilling to learn the basics of TD.
I don't play TD nowadays, but I used to a fair bit one at one stage. But I do remember the frustration of playing with AND against players who were defiant, argumentative, and unwilling to learn.
I would even try to help the other team by giving them a few fundamental tips just to make the game a bit more challenging. But nope, they NEVER listened. Some would even argue back and say, "we play this just for fun". Spamming effect towers, placing towers off-grid, having a disorganized defense setup, disregarding the economy, not communicating with your team members, and playing it as though it is a solo game rather than a team game is not something I would call "fun". On the other hand, epic TD matches with both teams displaying beautiful tactics, organization, and adaptability is (at least to me) fun.
Anyway, at that stage, I was telling some friends I played with that I wish we had forced tutorials or something to force people to learn just the basics of TD so the game can be a lot more challenging and exciting.