When I connect my Nexus 5 (Android 4.4.4) to Slackware (-current up through the kernel 3.14.12 update, I haven't applied the 2014-07-23 updates yet) I receive two alerts on the phone stating: "USB debugging connected." and "Connected as a media device. Touch for other USB options." When I select the "Connected as a media device." alert, I am given the options of having the "USB computer connection" connect as either "Media device (MTP)" or "Camera (PTP)". I usually keep the phone set to "Media device (MTP)". Does your phone give you these alerts, and is it set to act as a media device?
In this configuration, the phone shows up in Dolphin, under devices, as a portable media player. The phone model is recognized, and I have full access to the internal storage (from a user account).
My user account is a member of the "plugdev" group, which is necessary for access to some devices. I'm not sure if MTP devices require the user account to be in this group. Is your account a member of this group?
The two non-standard pieces of software that I'm using that may make a difference are "usbmuxd" and "libimobiledevice", which are intended to allow access to an iPod. They shouldn't have any impact on how the Nexus 5 is handled, but I haven't dug in to the internals of everything that these packages provide.
Hope this helps...