1) Damn...they really really really portrayed an amazing Tony in this movie. I think Mandarin(poster) will love how much he seems to have matured. He is still the wise cracker...but...ok the big spoilers start now....
He is married to Pepper is thinking of having a family and children and is still tinkering like the dickens because...
2)We get a new nanite armor that is freaking cool...maybe cooler than ANYTHING we have seen in the comic books, and it is HELLA powerful...making armors like the Hulkbuster look old and outdated.
3)We get to see Tony's fatherly instinct....his and Peters relationship really turns into a father son kinda of feeling. There isn't a lot of time for it...but you see Peter almost looking to him as a father figure and trying to obtain Tony's approval and you see Tony trying to do whats best for Peter.
4)Tony gets to go on a space adventure with Spidey sneaking aboard Thanos ship to try and save Strange who has been captured.
5)His fight with Thanos is stunning. He is with a team at the start but they pretty much all fall or are out of it for a bit and Tony goes man-o-mano with the mad Titan wielding 3 infinity gems and he holds his own...not for long, but much better than even the Hulk did when he fought Thanos using ZERO gems. Thanos also KNOWS who Stark is and seems to greatly respect him (we don't know why though). Then the trailers famous quote from Thanos "I hope they rememeber you."...I thought oh damn this is where we lose Stark from the MCU and we...(ok another massive spoiler).....
we DONT. Strange gives up the Time gem to save Tony who is angry asking "Why would you do that!!!" (he was willing to make the ultimate sacrifice to stop Thanos like the nuke in Avengers 1) and Strange replies..."It was the only way".
All in all...there isn't much more I think we could ask for in a x-over of this side for our boy. There are a few times he gets knocked around or looks about to be taken down. But he overall shines and more than just looks freaking awesomely powerful...he comes across as still funny but much more mature and adult and driven by his need to save others and protect the ones he loves.
A minor correction: when HULK attacked Thanos on the Asgardians ship, Thanos already had the POWER GEM that was locked up on Xandar thanks to the Guardians.
As to the rest, yes Tony was interesting in this movie and I hope RDJ really gets to go out with a bang as Avengers 4 is his last movie (unless a new contract happens)