When it comes to finding a "happy medium," ultimately elements of both shtick and substance are useful in attempting to appeal to an entire class or audience. With that in mind, I will go through qualities I've seen in my Rabbeim (of both mehalechs) that I found helpful, and others that I found to be nothing short of moronic.
a) The worst thing I've seen (shticky) rabbeim do: play favorites. In an attempt to be cool with the guys, the rebbi will kiss up to some of the cool/geshmak kids, and basically say "to hell with everyone else." I can't think of a bigger turnoff. I have been in two situations like this (and no, I was not the favored one), and boy, did it tick me off. I had rabbeim who had a few of the guys in the class at his house for shabbos, but wouldn't invite any of the quieter guys. Where's the consistency there? Now, some out there will say that I can't argue this one fairly, because I was once a victim of favoritism, but can anyone tell me what the argument FOR favoritism is? So a couple of kids might learn okay? I'll leave this one to the "cut off your nose to spite your face" crowd.
b) Another shticky no-no: schizophrenic rabbeim. What do I mean? You'll have some rebbeim who'll be acting shticky one minute, joking around and all, and suddenly the next second will be yelling, "okay, now we have to learn, enough!" Now, I'm not saying that any rebbi should either be all learning all the time or all joking all the time. A Rebbi has to be careful to draw a line, and let the talmidim know when they've crossed the line. But the main thing is, the Rebbi and the talmidim have to know where the line is! If a moody Rebbi comes in one day all relaxed and then gets really serious the next, then there's no point in the shtick. The kids will just be thoroughly confused, not to mention afraid to talk to the rebbi, because he might be having a bad day. A rebbi, more than almost anyone else out there, has to be consistent.
This is another reason that a rebbi has to be careful in getting too close to a buddy-buddy relationship. When I once saw a rebbi give a talmid a high-five, I thought that crossed the line to the point where I would find it hard for the talmid to take the rebbi seriously.
c) The delivery of the shtick has to be genuine, and not too over-the-top, in-your-face, or just plain artificial. The 10th anniversary of the OJ trial brought to mind a Rebbi I had at the time, and the great approach he used in discussing the event with us. He didn't try to come off as "look at me, I know who OJ is!" but rather was very down-to-earth about it, and discussed it as he would any mussar shmooze. Rabbi Paysach Krohn also does an excellent job interweaving his lines throughout his speeches. Funny or not, the jokes blend very well with the rest of his material.
Even a clueless grade school kid can see right through a rebbi that's as artificial as calorie-free chocolate syrup. In which case, the shtick simply backfires.
In a similar vein (with regard to substance), it is very important for a rebbi to feel passionate about learning. I can't forget the way my sixth-grade rebbi's eyes would light up when a kid asked a good kasha. And again, part of R' Paysach Krohn's success is the way he delivers his mussar with energy and sincerity. Real passion is contagious, but fake passion is simply pathetic.
d) Speaking of the Rebbi who discussed OJ, he proved the point that a rebbi can come off as being with-it without being shticky. This rebbi in particular knew how to walk the fine line of being with-it without shtick. These days, it can be quite important for a rebbi to show he's "with-it." Some kids find it remarkable that a rebbi can be so great and so with-it. (A friend of mine is an eight-grade rebbi in an immigrant school, and attributes part of his success (as opposed to some of the more beleaguered rabbeim there) to the fact that the kids think he's cool.) And it certainly helps in speaking with the kids and relating to them. And as that rebbi displayed so well, he can do this without lowering his stature or diginity.
In my early years of elementary school, I had a few old-school, European, long-white-beard rabbeim. They were decent rabbeim, but I could never imagine them dealing with older kids and all the issues they face. Try talking to them about your girlfriend.
e) Very often, a substance rebbi can be more engaging than a shtick rebbi. This has to do with pure teaching ability as well as presentation of the subject matter. A substance rebbi who can make kids think and make the material interesting will gather many more followers than a shtick rebbi with nothing to say. Also, a shtick rebbi can spoil the kids to the point where they're only interested in the shtick, not the learning. How a rebbi can be engaging in terms of his delivery and methods of teaching is a separate discussion.
One incident comes to mind: When I was in high school, one of my rabbeim decided to try a new shtick: ban all note-taking during gemara, in order to get us to think. In retrospect, I think the concept was an excellent idea (considering how nobody thinks anymore), but the way he presented it was awful. He took away notebooks from kids that were writing, and as a result kids just took notes from under their desks. And his shiur was simply not stimulating enough to make us think. So in essence, his grand plan ended up going nowhere and just ticked off a bunch of kids. This, I beleive, is an example of a great idea that was unsuccessful simply because it was very poorly implemented.
I believe that these are but a few principles (one can write a whole book about teaching) that separate the great rabbeim from the okay rabbeim. Like I said before, every kid will have a different rebbi that'll appeal to him. However, we clearly see that some rabbeim have been able to attract hundreds of talmidim, while others go back to business after a few failed years. So there are certainly some principles that have to be adhered to in order to be successful. And by putting together the positive elements of shtick and substance, any Rebbi will certainly be off to a solid start.