When I first started playing Ultimate - and now, still, in my league team Friskee, who are about as nonserious as can be - the highest conceptual level my defensive strategies operated on was pretty much 'turn the disc over'. Back then it was usually dumb running and athleticism (relative to old men in parks) that got me the turns.
As players get more serious about defence, the individual strategies become slightly more advanced: baiting your man, intentionally letting him get open, so that when he slows down to receive the disc you are there with the catch block, poaching - a new set of (admittedly riskier) defensive skills complemented by developments in dumb running technology such as not biting on fakes/jukes, recognising height/speed mismatches and calling switches.
The rolling stone gathers some more moss: gradually I became aware of zone and junk defences. These were put on in response to broader trends recognised in the opposition - rather than, say, calling a switch because your man is taller than you, we'd put on a zone because they were short on good handlers or were running us ragged on man. This is sort of the next evolutionary bit, where the transition was made from an individual defensive effort to a collective team strategy.
Where are we now? From what I've read and heard, we're nearly at the stage where, at elite levels, Ultimate is conceptually on par with other sports. Soccer has had its gameplay evolution (from one formation, 5-3-2, in the 1880's, to many, see this link) and continues to develop. From what (understandably) limited outlines I have been able to procure about various teams in terms of their offensive or defensive structure, Ultimate is no longer necessarily about who can run further or faster. Defenses have conceptual aims: recognising strengths in the opposing offence such as near-flawlessly executed set plays (combated by a defence that utilises clam, transitions and junks to dam the flow) or a highly developed short game running from a vertical stack(tips: defensive players clog slightly, making clean cuts harder, and counterbalance with a roaming defensive deep to ward off the easy huck) and, for lack of a better word, neutering them.
When it really comes down to it, Ultimate is simple. I have even supplied a flow chart for your viewing pleasure:
