The proportions.
Bike sizing is completely stupid. The nominal size of a bike (like 17") is a measurement from the center of the bottom bracket, along the seat tube, to one of about four points on the frame. It's not very meaningful because the seat post can be raised or lowered anyway. Someone has to be very tall or very short not to be able to get the right pedals-saddle distance on almost any frame.
To me, the most important dimension on a bike frame is effective top tube length - the length from the center of the intersection of the top tube and head tube to the center of where the top tube would intersect the seatpost if it was horizontal.
Most bikes with a given nominal size have about the same effective top tube length. The Flightline's top tube is about an inch shorter at any given size than that.
A lot of beginners are also sized in totally asinine ways - stand over the bike, see if there's enough clearance, get sold the frame. Clearance for what? Sitting on the top tube? Those of us who are right around average height or a little below don't get a lot of clearance from a modern front-suspended hardtail, unless it's something like a dirt jump or trials bike, so the result of that kind of sizing is getting sold a bike that's too small. A lot of beginner models, like the Flightline and Trek's 3-series, also have a shorter top tube than their manufacturers' higher end models.
If someone's not in very good shape or doesn't want to work very hard, he's likely to ride in a very upright position. Riders who pedal continuously ride in a lower position, to counterbalance the pedaling; the stronger they are, the lower that position gets, at least as flexibility allows. A very strong rider may actually pull up on the bars to counter the pedaling; even a modest rider does that in a sprint.
None of this necessarily applies to you. However, chances are that when you entered the sport, you didn't buy the size bike that you'd choose if you had it to do over. You can do it over if you want to, and riding some other bikes will give you an idea as to whether you want a different bike, or if you think your Flightline totally kicks ass, and you'd rather bolt the money to it.