Anything that exists in the standard CS:S maps won't be the cause. People have also been playing all kinds of custom maps for CS:S, HL2DM, etc. in gmod without problems. So it would have to be something non-standard.
Regarding the original questions:
1) CS:S player spawn entities will work fine in TTT.
2) Weapons and ammo, I suppose.
There is a bunch of mapping information on this page.
In general though, as long as your map loads, people can create a so-called "
re-arm script" for TTT that defines weapon and ammo placements as well as spawnpoints. This means you won't actually need to modify your map at all, anyone can create a script to make it TTT-playable in about 15 minutes. Of course, all this does require the map to load without crashing.