Matt Wilson wrote:
But that doesn't mean you won't get really irked. How much "borrowing" goes on among designers?

Well, since the vast majority of commercially-visible RPGs are all the same for the basic structure, and use, oh, maybe, 3 different mechanical models, i'd say that a lot of borrowing goes on.

Let's take an obvious example:
D&D3E has, really, not that much in common with earlier editions of [A]D&D--pulling a number out of a hat, i'd say about 20% of the mechanics come from D&D. However, it has perhaps as much as 50% in common with Ars Magica; i'm not familiar with RMSS, but i've heard that, among other things, the whole feat sub-system is taken straight from RMSS; large chunks of it also are highly reminiscent of Talistlanta 3rd ed, GURPS, and Hero System. Now, are any of these things actually borrowed (rather than convergent evolution)? I don't actually know. But the authors are pretty well-acquainted with the RPG market, and the authors of D&D3E were designers significantly involved with Ars Magica, RMSS, and AD&D in previous editions, which is why i lend the most credence to those similarities.

Likewise, V:tM is basically the setting of NightLife (with Goth substituted for Rock as the social slant) mated to the mechanics of ShadowRun--which is, in turn, basically a tweak of the Star Wars mechanics. And, while i have no idea if NightLife is actually derived from it, i'm pretty certain it post-dates the play-the-monsters supplement for Chill, and has a fair bit of tonal similarity, IMHO.

So, i won't address the attitude towards it, in terms of acknowledgement and credit, but i will say that it happens. A lot. To the point that the commercially-visible RPG market is sufficiently incestuous to look pretty homogenous, and leads to threads on EnWorld about not having seen anything "new" in a while, and not seeing anything "new" coming up.