(1) You get to have fun in rehearsals. Rehearsal can be really fun, but after a while, it can wear on you. There are often a lot of late nights, especially during "Hell Week" (or Tech week, the week before opening). And ask any actor how many times they've said "I can't; I have rehearsal."
(2) You get to show off. Performing for a live audience is an incredibly energizing experience, but beware of stage fright. Some tricks: don't look any audience member directly in the face; pretend you're still in dress rehearsal; picture them in their underwear (unless that makes you laugh inappropriately).
(3) You get to dress up. Who doesn't like to play dress-up? But you'd better take care of your costumes or suffer the wrath of the costume designer...and you've got make sure you can continue to fit in them, so no binge eating!
(4) You get to sing. This one is obvious, but you'd better be good at memorizing music, and depending on the role, not just big solos, but lots of harmonies and backup vocals. Plus, you have to stay in tip-top vocal health, including limiting your diet and using your voice sparingly.
(5) You get to be creative. The creative process is an actor's bread and butter, and it's a joy when you're allowed some free license, but you must also be good at taking direction and realize that the director's word is final (no matter how much you disagree).
(6) You get to play with toys. Props (and set pieces) are a blessing and a curse. They are important visual instruments of the action in live theatre, but you have to take care of them and remember how and when they come on, get used, and go off. Nothing is more terrifying than getting onstage and realizing you left it in the wings!
(7) You get to express yourself. Artistic expression is supremely satisfying, but you'd better make sure you are really connecting and exuding truth. If you're just going through the motions because this is the tenth (or hundredth) time you've done this show, you're not giving the audience what they've paid for.
(8) You get to make some really close friends. The theatre has a way of bonding people, especially since many actors and other creative types can be very friendly and open. But those relationships you build during a show almost always end abruptly after closing night. See you on Facebook!
(9) You get to play pretend. Just as with singing, acting is fun, but you had better be able to retain all your lines, blocking, and cues. Actors often live in fear that "today will be the day" they miss something important.
(10) You get compliments. (Hopefully). Getting positive feedback from audience members, fans, and even friends is certainly rewarding. But people can also be highly critical, especially when they are commenting anonymously on the internet. It's hard not to obsess over reviews, but you can always use them to inform your future performances.