If you see a brick wall as shown in the above image, then yes people will greatly struggle to upload from you. It will also have 'some' effect on your overall performance with LW. Uploading helps to find more sources & when searching, & to browse people sharing similar files. Some users might be able to 'force' uploads from you but many if not most would probably fail.

Normally in the Library window you'd see something like below (taken from 3 of the folders I'm sharing.) In the monitor window, in the upload section you'd see what files are being uploaded at that time & the ip's of such users & if you choose to show it, also the program client they're using (eg: Limewire, Phex, Frostwire, shareaza, morpheus, bearshare, Gnucleus, etc.)