The vast majority of the site is text.
I believe their is research which indicates that serif faces are easier to read for bulk text than sans serif faces.
Sans Serif is good for titles and labels.
So the two examples both work for me, but don't actually illustrate enough of the new look to make a decision.
EDIT:
The example doesn't show the 'All Questions' view which includes a line of text from the question. I don't think that is improved on the other stack exchange sites by making it sans serif. However, I'm not going to be as sad about that as much as my sadness if you change to sans serif for body text in questions and answers.
There are people for whom 'looking tidy' is more important than easy to read or use. There are also people who equate 'modern' with better, and 'modern' with sans serif. I am not one of them.
IMHO you have a horrific job, 'everyone is an expert'.
I don't like the sans serif font for body text on e.g. the Arduino stackexchange. I do like the question titles in sans serif.
San serif titles, serif body text looks pretty good.
For an example, look at Horowitz and Hill's "The Art of Electronics"
I like reading that book. It is easy to read and easy to navigate. When you look closely, you'll see that it is set with two different faces, and the subtle difference helps to structure the text, while not being too 'shouty'.
An argument for sans serif because answers might contain code seems weak.
There is the code markup, which is easy to find and use. So IMHO, that is a non issue.
The logo
This should be simple and bold, but preferably look like something 'meaningful'.
So not a zig-zag thing, which looks like an attempt to be 'cool' but without working the idea through to illustrate meaning. It looks more like a component being hit by lightening, or a warning symbol. Neither of which seems positive or welcoming.
It could be a real schematic symbol or illustration of electronics. I would not recommend a photograph of electronics as being not abstract enough, too 'busy', and may end up being too specific.
I suspect most new arrivals are not electrical engineers, so a 'Fritzing'-like illustration might be more welcoming and friendly than an electrical schematic symbol. So something as simple as colour coded resistor and LED 'Fritzing' illustration might work.
On the other hand, a simple schematic illustrating power and a load, might encourage people to illustrate their questions, and give us the important information :)
I would probably not use a BJT.
IMHO a lot of the ubiquity of electronics is driven by humanities ability to make lots of MOSFETs very cheaply. Hence that seems relevant. IMHO, other keys are SMD and PCBs. I think PCBs have been 'done to death' on other sites already though.
Edit:
If you use a MOSFET, please consider using a useful, or 'proper' symbol; like Horowitz and Hill "The Art of Electronics", I don't like MOSFETs which carry so little information that I can't tell whether it is P-Channel or N-Channel. However, IMHO that is a small detail.
Please try to avoid creating a taller logo and buttons than the current one. I like to have the maximum useful information in a window without needing to scroll. I quite like your new design with logo and button on the same 'line', as long as I don't have to scroll right to get at the important 'Questions' button.
Vertically Scrambled numbers ("testing, testing 1234567890")
I believe the typographic intent for having numbers have different height is to aid readability. There is lots of evidence that ALL UPPER CASE, or single height, is much harder to read than Mixed case, or even all lower case. AFAIK, this is wired into human visual systems, our eyes look at outlines, where UPPER CASE has very little information to help recognise the word, where as lower case often has more, with ascenders above the average height and descenders below the line.
Also, the slight dip below the base line of the digits was intended to enhance the character so that it gives the impression that it is sitting on the common baseline.
I don't have a problem with digits having slightly different heights and descenders if it makes sequences of digits easier to read. Used tastefully, it can help parse numbers into visual groups, reducing the chances of reading errors. As a contrast, credit cards resort to extra spaces to improve the readability of single height digits in the card numbers.
However, IMHO the font used here seems to have gone a bit too far down with its descenders. Pulled back about 50% might look less jarring. I suppose if people really object, they can use the 'code' or math markup to write numbers, though I would prefer the normal body text font to do a less intrusive job. IMHO good typography should be 'invisible', and other visual clues communicate the 'brand'.
