I went from the S3 (stock, unrooted) to the S5 also.
I do see some improvements, like the screen, for example, which is noticeably better. It's also a bit bigger, which is nice, as I'm getting old and can't see as well as I used to
I had the 3000mah extended battery on the S3, and it seems to me that the S5 has about the same battery life as the S3 with the extended battery.
Not sure I see much of a speed difference. The S5 is maybe a tad faster, but in day to day use, I don't really notice all that much of a difference.
In general, I do think the S5 is a better phone. But, I do agree that the S5 is not a revolutionary change. In fact, in a lot of ways, it seems just like my S3. That's not necessarily a bad thing, as the S3 was a great phone.
I made the move to the S5 mostly because I wanted the fingerprint sensor. It works pretty well, though I would argue not as well or as refined as the sensor in my daughters Iphone 5S. But it does the job, and overall I am happy with it.
I did buy an aftermarket Qi wireless charger, which is a neat trick also, but I believe they were available for the S3 also.
In summary, I'm happy with my purchase. Not revolutionary, but a decent evolution of the S3.