I don't think I'm adding any new points, but here are my lessons anyways:
Lessons that hurt
-Watch the rewind knob as you load the film. A small mistake at loading time results in 36 "missed" shots, at lease of few of which you'll be certain would have been your best shots.
-Even good labs make mistakes (like stapling your negatives, right through your favourite frame). Learn to process your own B&W, so when shit happens you have no one to blame but yourself.
-I've had two rolls of CR200 fall off the spindle inside the cassette. I'd love to say the resulting images made the recovery effort worthwhile, but they really didn't. If you're going to shoot slide film, pony up for the good stuff.
-Some labs that do enlargements from negatives are just scanning and then printing on cheap thermal dye printers. Nothing worse than seeing jpg artifacts in your 11x14 "film" enlargements.
Lessons that make it better
-A negative is a half way point, not a finished product. With a $200 scanner you get results comparable to a DSLR from 2007 (that would probably cost you less than $200 today). But, with an enlarger, you get magic. If you can't set up your own dark room, look for a community dark room, or failing that, find a printer who still uses wet printing and have them print your favourite negative, it'll be worth the money.
-RA-4 Metallic paper is amazing when done right.
-If you don't have a good way to view slide film, don't bother. Shoot Ektar have have it printed on RA-4 Metallic paper - it's not the same, but it is bloody brilliant.
-Get a bulk loader and load your own film. It'll cut your film costs in half and takes minimal effort.
-Sometimes cheap film is what you want. I love Porta & Ektar, but for small prints Fuji Superia is a great colour film and cheap as chips. I find Kentmere harder to load onto reels than HP5+, but once printed, I couldn't tell you which was shot on which.
-If you have access to an enlarger than can handle it, try Medium Format. 645 is much easier to print than 35mm and looks stunning at 8x10. 645 and 6x6 cameras are inexpensive and fun to use. Everyone should give it a try once.
-Learn the sunny 16 rule using cheap C41 or 400 speed b&w. Once you learn to guess what your exposure should be you open up a whole new world of light, portable, and fun cameras to use. Film is far more forgiving than you expect.
-Play with off camera lighting. Even a single 30 year old speed light and a 15' PC sync cord make your portraits ten times better than flash shot from the hot shoe.