When the path to the Steam library contains certain special characters, things start behaving weird, and with different behavior depending on which character it is.

If the path contains a colon, then some games will fail to launch. The games I've tested which fail to launch are Portal, Portal 2 and Garry's Mod (all Source games). Portal and Portal 2 fail with only this error in the terminal:

/mnt/ext4_data/Games:test/SteamLibrary/steamapps/common/Portal/hl2_linux: error while loading shared libraries: libtcmalloc_minimal.so.4: cannot open shared object file: No such file or directory

and GMod fails with this error:

AppFramework : Unable to load module /mnt/ext4_data/Games:test/SteamLibrary/steamapps/common/GarrysMod/bin/filesystem_stdio.so!

so it seems to be related to the libraries bundled with the games. Meanwhile, Super Hexagon and Antichamber(both non-Source games) run fine.

If the path contains a double quotation mark ("), then the Steam client says "New Steam library folder must be on a filesystem mounted with execute permissions". The file system does have execute permissions, since Steam games run fine off it when the path contains no special characters.

A parenthesis in the path yields no problems. I didn't check other characters.

I'm running Fedora 21. This is a real problem because my system by default mounted my external hard drive under /mnt/usb-Seagate_Expansion_Desk_16BG347457KX-0:0-part1/, and the colon in the name caused some games to fail to launch. Changing the mount point worked around the issue.

System Information:

Processor Information:
    Vendor:  GenuineIntel
    CPU Family:  0x6
    CPU Model:  0x3c
    CPU Stepping:  0x3
    CPU Type:  0x0
    Speed:  3900 Mhz
    8 logical processors
    4 physical processors
    HyperThreading:  Supported
    FCMOV:  Supported
    SSE2:  Supported
    SSE3:  Supported
    SSSE3:  Supported
    SSE4a:  Unsupported
    SSE41:  Supported
    SSE42:  Supported

Network Information:
    Network Speed:  

Operating System Version:
    "Fedora release 21 (Twenty One)" (64 bit)
    Kernel Name:  Linux
    Kernel Version:  3.19.7-200.fc21.x86_64
    X Server Vendor:  Fedora Project
    X Server Release:  11603000
    X Window Manager:  Xfwm4
    Steam Runtime Version:  steam-runtime-release_2015-01-06

Video Card:
    Driver:  NVIDIA Corporation GeForce GTX 760 (192-bit)/PCIe/SSE2

    Driver Version:  4.5.0 NVIDIA 346.59
    OpenGL Version: 4.5
    Desktop Color Depth: 24 bits per pixel
    Monitor Refresh Rate: 60 Hz
    VendorID:  0x10de
    DeviceID:  0x118e
    Number of Monitors:  2
    Number of Logical Video Cards:  1
    Primary Display Resolution:  1280 x 1024
    Desktop Resolution: 3200 x 1200
    Primary Display Size: 13.31" x 10.63"  (17.01" diag)
                                            33.8cm x 27.0cm  (43.2cm diag)
    Primary Bus: PCI Express 16x
    Primary VRAM: 3072 MB
    Supported MSAA Modes:  2x 4x 8x 16x 

Sound card:
    Audio device: Realtek ALC892

Memory:
    RAM:  16019 Mb

Miscellaneous:
    UI Language:  English
    LANG:  en_US.utf8
    Microphone:  Not set
    Total Hard Disk Space Available:  92089 Mb
    Largest Free Hard Disk Block:  4460 Mb

Installed software:

Recent Failure Reports:
    Sat May 23 23:26:04 2015 GMT: file ''/tmp/dumps/assert_20150524022559_19.dmp'', upload yes: ''CrashID=bp-dd88fa6a-a61d-4f95-b32b-38f512150523''
    Sat May 23 23:29:38 2015 GMT: file ''/tmp/dumps/assert_20150524022937_6.dmp'', upload no: ''HTTP response code said error''
    Sat May 23 23:42:11 2015 GMT: file ''/tmp/dumps/assert_20150524024210_6.dmp'', upload no: ''HTTP response code said error''
    Sun May 24 10:30:54 2015 GMT: file ''/tmp/dumps/assert_20150524132944_53.dmp'', upload yes: ''CrashID=bp-7b00372e-8cd0-41b5-b1a5-2ac242150524''
    Sun May 24 10:32:06 2015 GMT: file ''/tmp/dumps/crash_20150524133058_63.dmp'', upload yes: ''CrashID=bp-0867cfa3-5215-4ff0-8b01-e96cb2150524''
    Sun May 24 10:32:13 2015 GMT: file ''/tmp/dumps/assert_20150524133211_18.dmp'', upload no: ''HTTP response code said error''
    Sun May 24 11:19:40 2015 GMT: file ''/tmp/dumps/assert_20150524141938_18.dmp'', upload no: ''HTTP response code said error''
    Sun May 24 11:27:08 2015 GMT: file ''/tmp/dumps/assert_20150524142604_18.dmp'', upload yes: ''CrashID=bp-239a518a-e38f-4b26-80f7-c9ab02150524''