Looking for a good scientific drawing software

Do you want a generalized vector graphics editor or do you want a dedicated diagram editor? If you want a generalized vector graphics editor, then you will be hard pressed to find a better application than Easy Draw 4.0. If you want a diagram editor, then OmniGraffle Pro 5.3 is your best bet.

Haven't used it in a while but there is an app called Concept Draw...

http://www.conceptdraw.com/products/office/whatis.php

Omngraffle is nice.

Last month I decided to give a few drawing apps a trial, as it had been a while (until recently, I've had Illustrator... didn't install it on my new iMac, though). Want something to replace it, but it will only be used from time to time and needs to be easy to use and inexpensive. Have already plunked plenty of cash into apps.

Didn't like Easy Draw for the same reason: it wasn't so easy and user-friendly.
Intaglio was deleted almost as soon as I opened it for the first time; almost didn't download it anyways after looking at the screenshots.

Can't remember the other two or so I tried, but nothing fit the bill.

Most of my drawing is hand-sketches and I have a nice app or two for those, but they won't do what you're looking for.

Really is a bummer that there just are any "nice" and "easy" vector apps out there that don't cost a bundle.

How about Omnigraffle plus this?

I don't own a license for Eazy Draw, but it appears to have everything that you asked for--and a few things that you did not ask for. Of the latter, are mathematically-defined curves. Of the former, it includes an arrow editor.

As for its ability to edit PDFs, my guess is that it does about as good as can be done. Of the many things that you need to understand about PDFs is they were never intended to be edited. PDFs are also not object oriented, which can often turn even minor single modifications into disasters. My experience with PDF's sibling format, ESPF, is much the same for much the same reason. PDF is a distribution format, not a source format. If you want to edit PDFs, then you are better served by editing a source file and then printing [or converting] it to PDF. If you want to edit PDFs from other authors, then you just have to do the best you can.

I am in much the same position as you. It is one reason that I continue to use MacOS X 10.4 on PPC-based Macs. This allows me to continue to run Canvas X natively. It runs via Rosetta on Intel-based Macs on MacOS X 10.6, but not at all on Lion. It is frustrating that I had more better choices in 1991 than I have now in 2011, but I digress....

At this level, nobody is going to pre-chew your food. Put on your big-boy pants, jump into this with both feet, and get to work. If you are unwilling to learn a new application, then you should hire a friend or colleague who is willing to learn.

Have you tried PixelMator? It's quite nice!