I've just stumbled on some big problems with IE11 and Notes too, specifically opening a mail db. Both StdR8Mail and StdR85Mail fail in grand fashion with IE11.
Just as a side note, we've found problems with another (unrelated) app, and I tracked that issue down to the UserAgent string that IE11 returns. IE9 (don't know about IE10) includes "MSIE" somewhere in the string, IE11 does not. So in this case, the browser-specific code fails because it thinks it's talking to FF or Chrome and not IE. I'm wondering if something similar is going on with Notes.
Lotus Notes and IE10 Compatability (Narsimha Rao 3.Dec.13)
Feedback response number WEBB9E3JBN created by Duston C. Suits on 12/04/2013
. . Talk to the developer (Karl-Henry Mart... 3.Dec.13)
. . Lotus Notes and IE10 Compatability (Duston C. Suits... 4.Dec.13)
. . . . What about mail9.ntf ? (Karl-Henry Mart... 4.Dec.13)