There are three I would have liked - Redpath, Tompkins and Polledri.
Redpath - was in England age groups and England senior squads, Scotland were just willing to cap him first (and may well have been his preference anyway). I think he was capped before he had earned it (just about ready, but not proven), but he took to it like a duck to water.
Tompkins I just don't get, nothing about him, at club or international level, says that he's an international quality player. I just don't see it in him. I'm more upset by losing Jonny Williams to Wales. Neither were ready for international rugby when first capped by Wales.
Polledri, there's a much better call for, as he actually was doing enough at club level to deserve a shot at that level. Though we weren't crying out for an 8, or any backrower at the time, and IIRC hadn't even given up on Hughes yet by that point, whilst Mercer and Simmonds were there or there abouts, and Jack Willis was already being talked about as the best of the 6. For England, he'd have been 1 of 6 options, whereas for Italy, he was pretty much automatic first choice.
I'm not going to blame any coach for not calling a dual-nationality player who isn't really ready yet just to stave off the poachers. Wait till their ready, and then have that conversation.
I'm also not going to blame a coach who says "if you're happy playing for a different nation, then you're not dedicated enough to this country".
Of course, everyone's milage will differ.