But, it's not my company; and what I can and can't do could change instantly if the guy at the top of the org chart decides on a whim that things are going to be done differently. While I don't think it's likely to happen, it's still a possibility; and you have a lot less of that when it's you at top of the chart.
[Edit: I just want to preempt some other angles if I can; I know that things get muddy when you deal with investors and and boards of directors, and when you deal with partners and staff and all of those things. There's a bit of nuance here that I'm somehow not conveying. I guess at this point you either get what I'm saying or you don't.]
That hunger can easily become real, not virtual, if you're not careful :-)
Things take longer than they seem. Money lasts shorter than you plan.
Like I said before, I'm not one to discourage, but OP doesn't even have a "hair on fire" idea yet. I'd think twice before I left a job I loved.
(Also, I totally agree with you about having an idea to work on before quitting.)
I've loved all my jobs.
Also, don't discount all the data you get from your job to feed your startup plans.
I wouldn't dream of starting a competitor to the project we built together.
How about this:
Find some idea, get your friend on board as an investor / advisor ? You guys seem to get along well and have each others interests at heart otherwise you wouldn't say you loved the job that much.
Three birds with one stone, angel investor / mentor / continued professional relationship, only this time you are in charge.
What are you crazy?!?
I've had many jobs and I've liked few of them. If you have a job you love working for a schoolmate, why would you want to leave? You don't even have any plans. I'd understand this a little better if you had a project you're dying to work on full time, but that's not the case.
Why don't you just keep your job and find a side project. If that side project gets big, go part time. If it gets so big, you're burning to work on it full-time, then quit, but not before.
Good jobs are hard to come by and jobs you love are almost impossible to come by. Also, don't discount all the data you get from your job to feed your startup plans. Lots of people would love to do a startup, but don't know what to work on. People with jobs don't have that problem as much. The job can be the source of lots of great ideas for things people actually need right now.
I'm the last person to discourage anyone from doing their thing, but job vs. startup is not a binary decision. You can do both, at least for a while.
Keep that all-time favorite job for now. You can always leave later once something else has wings.