SF units, tier one in particular, will pay an increasingly important role as the asymmetrical threat becomes ever more complex, and clearly defined front-lines cease to exist. Such units must be able to operate in both a green (conventional) and black (urban/ plain clothes) environment. They represent the sharp end of the much larger intelligence community to undertake missions of strategic importance, such as the destruction of critical infrastructure and weapons of mass or the assassination of leaders / high-profile terrorists in difficult to access environments.
WWII well known SF units (LRDG, early SAS for example) tended to be populated by intellectual's and adventurers supported by nonconformist troops who didn't fit in with typical regimented routine. The reason for this was it took time to establish formal selection processes, senior officers assigned to form/command such units searched for mountaineers, linguists, explorers and prominent sportsmen. As the units developed they would look for nonconformist hard-men who enjoyed a challenge who could complete a ad-hoc series of tests.
Today tried and tested selection processes find men /women with similar capabilities, individuals who relish the opportunity to push themselves physically and mentally above and beyond the average infantry unit. These individuals may no longer be classic scholors, but they tend to exibit high IQ's complimented by bucket loads of common sense.
There also exists today a tiered system, whereby SF units are rated differently and often geared towards a single mission. Tier one are often the jack of all trades collective, with the largest budgets and range of capabilities (land, air, mountain, mobility, water), whose primary mission is to provide the state with a strategic go anywhere hard-hitting precision asset, such as: US Delta, Dev-Group and UK SAS/SBS, Second tier SF assets have been created to support theses, these may undertake a specialist role, such as securing the outer perimeter of a target thus freeing up tier one to focus on the primary target, or to gather ground intelligence in the run up to a tier one strike. Examples of such units would be the US Rangers and UK SFSG / Special Reconnaissance Regiment.
The advantage of having a tiered system is you can provide a scalable career path for interested parties. Join the infantry for a few years to gain the basic skills and experience, undertake selection for a tier two unit and gain specialist weapons and trade training before going for the final an ultimate challenge - tier one selection. By taking this approach you should end up with tier one team members who are of the right age bracket (mid-late twenties) and who have a lot of operational experience under their belt. I've never been a fan of the direct entry approach unless the individual brings a specific skill to the game because IMHO there is no substitute for operational experience.