After all, Guile is the quintessential TZW character. What makes him tick? In a word, (internal) balance.
In any given combo scenario, Guile always seems to have multiple options but there's always some sort of tradeoff involved. His Flash Kick, although not an exceptional combo tool by any means, is significant in the sense that it connects from well outside the maximum Sonic Boom combo range. It adds another layer to the number of charge moves Guile can fit into a combo. Each of his possibilities comes with built-in limitations, which provide the player with countless challenges to overcome, which in turn result in interesting combos. They look impossibly outlandish to beginners but more importantly they challenge the expert players' perceptions of what's possible in a game they've been playing for years. Because let's face it - it's not very difficult impressing anyone new to the game or anyone with a short memory. Impressing the veterans demands quite a commitment.
For example, take a look at his light attack arrangement. His jabs have identical range so they're equally useful in different situations. His standing short does more damage, but it's a couple of frames slower. His low short has the most range and inflicts the most damage, but it's the slowest of the four plus it's not cancelable. There's always a decision to be made and it always ripples throughout the rest of the combo.
With Guile's F+HP backfist range, there's always something he can do after a Sonic Boom connects. But it's rarely ideal because it ends combos without much excitement. Instead, Guile can walk up and do something more impressive.
And of course, the Sonic Boom forms the core of Guile's arsenal. It opens up countless possibilities due to its exceptionally quick recovery, but also presents countless challenges due to charge issues. Most characters don't have that extra dimension to their gameplay. Even charge characters like Honda and Blanka rarely use charge more than once per combo.
ST Guile's super does the highest number of hits in the game, tied with Chun Li. But it takes a lot of work to make the FK super connect, especially if you want to get all six hits. Any combo into FK super is flashy, but there's actually a lot of room for crazy madness.
Finally, Guile has to deal with dizzy limits more than any other character in the SF2 series. Most special moves in the game either knock down or end combos. Guile has to choose his attacks wisely to put three or four Sonic Booms in one combo. Sometimes even redizzy limits aren't high enough to explore the full potential of a setup.