To me, the issues Matt mentions - how true which one is to the Church established by Christ, was key. And I'm sure he can do a better job of detailing that.
Further, as I have learned more, I see undertones of how we approach spirituality being affected by some doctrines that differ between us.
Among the most important of these for me are the more legalistic view of sin that Catholics have, paired with the idea of created grace (which makes it quantifiable).
It's VERY tempting, because it's comforting, I'm sure, to go to a priest for confession, to be given a quantified penance, and be told you are "forgiven". Of course we also go for the Sacrament of confession, and receive absolution. But I think with Catholics I get the idea of a slate being wiped clean over and over, with a dire fear (damnation) if you happen to die with the wrong kind of sin not yet forgiven by a priest.
In Orthodoxy, there is a purer understanding, IMO, of ourselves as fallen creatures, attempting with all the ways God offers us, to cooperate with His grace - His very energies - HIMSELF (in a sense) - in order to be transformed, restored to both the image and likeness of Christ, which was marred in the fall and through our sin. Simply - we are sick, and we need a spiritual Healer to make us well.
The Church has these medicines, tools, as well. There is far more understanding of spiritual guidance in a personal sense, I think. And comparing the fasting, as just one example. (One thing that almost made me not want to be Orthodox - again, that's a rather unimportant "personal preference" thing and not worth considering. Yet now that I really can't - I miss it more than you could know.)
Little things in that. Catholics can choose to "give something up" for Lent. Orthodox understand this is self-direction with its own pitfalls.
Other little things - Catholics are encouraged to visualize during prayer, things which excite the imagination and can lead to ecstasies. The Fathers understood this as dangerous and warn against it.
Such customs (I don't know if these are actual Catholic doctrine but seem to be?) that border on superstitious imo, such as wearing certain colors of scapulars? Either way they betray a certain "grace for favor" mindset that doesn't sit well with me.
There are good things in Catholicism. After all, we share a certain foundation. But those areas where we differ point me toward Orthodoxy (again, the bigger dogmas and developments are the more important - the papacy, papal infallibility, the immaculate conception, created grace, purgatory, the Filioque, etc. - these should be examined in a historical context).
