Natural stones, such as travertine (a type of limestone) and slate, and denser stone, like granite, are more porous and less impervious than porcelain and glass-based tile. Because of this, a basic, readily available polymer-modified thinset bonds well to natural stone and the substrate (setting surface) inside a shower. Natural stone installation in a shower does not require use of the more expensive "flex" or specialized material-specific thinsets.
Although not specifically required, traditional ceramic tile does not require the usage of polymer-modified thinset. Thinset such as Custom Building Products' Customblend works well with ceramic tile. For extra bond, acrylic mortar admix can be added to the standard thin-set mortar.
Porcelain tile -- due to it's dense, impervious body -- requires the use of specialized or "modified" thinset. Polymers such as latex are added to the thinset as it is manufactured, giving the product the ability to bond to porcelain tile. A high-performance fortified thin-set mortar is recommended when installing porcelain tile.
Like porcelain tile, glass mosaics require the use of modified thinset. Look for thinset that is specifically manufactured for porcelain and glass tile installation. For added security, building product companies now produce glass-mosaic-specific thinsets. The Laticrete company makes a glass tile adhesive thinset.