Whoop! Accipiter beat me to the punch...but here's some add'l info on the 'poor construction' factor.
Actually, one of the reasons the Titanic may have been so badly damaged by the iceberg - an event, by the way, that it was ostensibly designed to handle - was discovered in recent years. Careful analysis of metal samples brought back from the hull of the wreck showed that the steel used in its construction has several characteristics that rendered it unable to handle the situation.
For one thing, the steel has very high sulfur and oxygen content, along with low silicon. This means that it was only partially deoxygenated (or, in the parlance, semikilled). Steel is deoxygenated in order to improve its crystalline structure and strength as well as its ductility. An article in the Journal of Metallurgy (JOM) notes that steel processes in Britain around the time of the Titanic's construction used acid-lined open-hearth furnaces as well as basic-lined ones. The basic-lined furnaces would have reacted the basic liner with the impurities (sulfur, etc) and removed them; the acid lining failed to do so.
Another finding is that the steel from the Titanic hull was 'banded' fairly severely in the longitudinal axis. It is hypothesized that this happened due to the steel being hot rolled into plate; the deformation at high temperature caused the crystalline banding to occur perpendicular to the direction of the rolling process. The steel, therefore, has a much better performance under stress in the longitudinal direction; think of that packing tape with plastic threads in it. Although it is easy to separate it into long strips, it is extremely difficult to tear across. In the Titanic's case, the 'threads' ran along the hull from front to back. As we see in the iceberg damage, indeed, the damage to the plate is in a rough line from front to back which is fairly narrow. The hull buckled across the transverse lines.
The steel, chemically, appears to not have been dramatically sub-par for the era. However, the primary problem that the high sulfur, phosphorus and oxygen content causes is a shift in the ductile-brittle transform point. This is the level of heat energy in the steel at which the metal shifts from being ductile (bending and then tearing under stress)) to brittle (cracking under stress). This is expressed as the temperature at which this shift occurs. It is always possible to fracture the steel, but above the transform point it requires a substantially higher and faster energy input.
Modern steel shifts from ductile to brittle at approximately negative 25 to 30 degrees Celsius. This does not include special application steels, such as the HY100 rated steel used in submarine hulls, but the more general purpose steel used in ships, structures, etc. The Titanic sample, however, shows that it has a transform point of around 32 degrees for the longitudinal-cut sections and 56 degrees Celsius (more than 100 degrees Fahrenheit!) for the transverse sections (showing the effects of banding). Ergo, at temperatures below these points, impacts were much more likely to cause fracturing than ductile bending and tearing.
The water temperature the night the Titanic sank was estimated at negative 2 degrees Celsius. Draw your own conclusions.
Information for this writeup was drawn from the Journal of Metallurgy (50,1,1998), as well as various websites on the Titanic sinking. The assemblage of the information is my own.