First, no assigned seating. So if you want a seat that doesn't suck, you have to line up a bare minimum of 30 minutes before the flight boards. I have better shit to be doing than standing in line to get on the fucking plane.
But what's perhaps even worse is that SouthWest attracts the absolute worst of travelers. Case in point:
On the AUS-LAS leg, the asshole next to me felt it was ok not to bathe or brush his teeth. I got to enjoy his BO every time he got even remotely close to me.
The asshole in front of me (who was pushing 300lbs) had ants in his pants. Every 5 minutes or so he woudl bounce all around his seat.
What's interesting is that neither of these rocket scientists could figure out how to recline. The guy next to me actually kept trying to recline with my button.
On the LAS-SAN leg, I was surrounded by noisy kids. Literally over a half-dozen (though at least not one family). At least that was only a 1 hour flight.
Now, if this was the only flight I was taking this year, and I was on my way to a vacation, it probably wouldn't have mattered that much. But when you're flying once a month or more, that shit just sucks.
Contrast this to a slightly more expensive airline where you can relax before boarding and don't generally have to deal with assholes or snot-nosed kids (since they're all on SWA). And where if you fly that airline often enough and book on the right equipment, you're essentially guaranteed an upgrade to first class (and if you've never flown first, you really don't know what you're missing... but I expect it's close to what flying used to be 40 years ago).