Have you guys also come upon certain tunes that are real “catchy” and that you like and “learn” relatively quickly but have the darndest time learning to play WELL?
A few of my own examples:
Banish Misfortune
Tommy People’s Jig #1 (from Cathal McConnell book)
The Templehouse Jig (Michael Coleman)
Okay, that’s in my own estimation; an objective observer (or listener) might say I haven’t learned to play anything well…
Philo
Two from me,
Tara Diamond’s version of ‘Sheep in the Boat’, simple flowing tune
but drives me 
and ‘killavil’ I don’t think I’ll ever be able to play it even approaching this
http://www.youtube.com/watch?v=OwZ9dFUSDCs.
nick
For me the Kesh jig qualifies. It’s the first tune I learned and I’ve played it in many sessions, but I can’t say I do it really well.
Also, I was just talking about this to a session mate, it’s easy to get bored with a simple tune like Finnish Polka and not work on it any more because it is so simple. But then you hear what Joanie Madden and Eileen Ivers do with that tune and you realize that it isn’t the tune that’s boring…it’s the player (me). I know people who won’t play Sean Ryan’s Polka because they are tired of it, but in reality I don’t think they can play it very well. The basics of tone, rhythm, articulation, lift… are harder to master than fingering some runs in a fast reel, at least for me.
Before I even opened the thread I was going to say Banish Misfortune.
Brian Finnegan plays this tune, and although very simple i just can’t put the same expression into it as Brian does 
I wish I could play it but he looks bored to tears.