As a beta reader, I would have to say that I will read pretty much every type of fic out there, but I do have more expertise for some types of genres and series. If it's a book/Anime/Manga series, movie, or videogame I'm quite farmilliar with, I will be much more critical than with something I'm not farmilliar with.
If asked to beta-read a fic of a topic I am unfarmilliar with, I may ask for a background of said topic and do a bit of reaserch before so that I will have a better understanding of the fic and will be able to helo you out to the fullest potential I can. ^^
I try to make it a habit to spell and grammer check my work, of course occasionally I will miss things, and I may do the same with the work that I am beta-reading. As I read I plan to make notes on things that can be improved, a suggestion of words to be used in place of others. For example:
"HELP!" Lily shouted as she ran down the hall and the zombie following her.
I may suggest you write...
"Somebody! Help me, please!" Lily screamed in terror as she ran down the hall. The zombie limped behind her, the smell of decaying flesh emitting from it disgusting form.
Comparing the first example to the second, the second is better than the first in the fact that it
* Is much more descriptive
* There is a better selection of words
* It gives the reader a better image than the first
Of course, you do not have to choose to listen to my suggestions if you do not wish to. I simply wish eto give you that option. The more descriptive your writting is, the better you will be able to keep your reader's attention. I will take into account what sort of audience your work is directed at as well, which will give me a better idea as to what advice to give you for your fic. Because if your story is directed at K level readers and you have it written at an M level, there are going to be some serious problems.