I came across your question while looking for the same answer! Only 3 years late…
Pride is, in my opinion, a deeply egocentric concept (which is why it's often seen as negative, certainly Google's definition speaks exclusively about self-satisfaction). The difficulty I've had in finding an alternative is that the feeling I wish to express when I come across this issue is still very egocentric: I feel good in your reflected glow (to paraphrase your earlier comment).
I've experimented with "I'm impressed by you(r work)", but this implies that you held a low opinion of them beforehand, and herein lies the problem: we're trying to express a relative feeling, maybe something like "your work is better than I've seen before", but this doesn't sound genuine at all.
I'm coming round to the feeling that it's the specificity of "proud" — parental, ownership-based relativism — that makes it work in this context at all, so if we wish to remove that aspect we might find that sense of genuine praise in picking something different, and maybe individual, to be specific about.
Rephrasing in a mildly deferential tone, perhaps referencing more of the effect the impressive aspect has had on you, might smooth off any remaining sharp edges:
- I'm in awe of your insightfulness
- Your attention to detail humbles me
- I'm envious of your ability (if you want to keep a deadly sin in there 😉)
- Your passion is inspiring
Of course, you can always flip the subject with a bit of linguistic gymnastics:
- I hope you're proud of what you've achieved, I certainly would be.