Since most people are only driven to write a complaint about a comment from the flight deck they only half heard and have completely misinterpreted, or that the cabin crew didn't do a service on a very short flight, it would be nice to balance these out with some complementary letters for a job well done.
Guess I'm not most people then. On the last 3 out of 4 flights I asked for a feed-back form to praise the crew (Emirates, all 4 flights. On the 1 I didn't ask for a form I slept all the way. Pretty hard to appreciate good service when you're sleeping). Tipping a crew member, though, is not ever going to happen; I'll never forget they are primarily there for my safety and not to mix my Martini's, and I'd no sooner tip a fireman or police officer than I would a crew member. Thank them, yes. Point out their excellent performance to their superior, absolutely. But I won't insult them by throwing cash at them - that's just vulgar.