Note: I'm not a historian but I'm trying to answer from my Swiss high school education.

According to [1], Switzerland is a special case (Sonderfall). Indeed I think that Switzerland's history is a bit hard to compare to any other country in Europe, mainly because it got rid of its monarchy so early - the 'Eidgenossenschaft' rebellion against Habsburg formed in the late 13th century already (date not well documented), introducing a Republican form of governance in the 'Tagsatzung' with representatives from each region - and even democratic elements with many city councils being elected by the citizens. The direct democratic cantonal assembly 'Landsgemeinde' [3] is documented back to the 16th century. Even at the start of the 20th century, France, Switzerland and San Marino were the only non monarchic countries in Europe [2]. With France having their people's revolution in 1789 and San Marino being founded as a monastic republic early on, I would see the 'Eidgenossenschaft' (Switzerland's predecessor which can be seen as the basis of its identity) as the earliest European Republic formed by 'commoners'.

More and more city states around the 'Eidgenossenschaft' seem to have found this form of governance attractive and joined the confederation - either as a full member (e.g. Berne, Lucerne, Fribourg) or as an ally (e.g. Geneva, Republic of Valais) [4]. These city states usually brought in their surrounding rural areas as 'Untertanengebiete' (tributary areas with no or very limited political power), but the cultural centers mostly joined from free will, which is I think one of the most important aspects of how and why Switzerland has formed its identity - it was an alternative to the dominant monarchies.

Now obviously there were multiple wars fought by the Swiss against attempted invasions of monarchic armies [5]. The fact that the confederacy was effective in protecting confederates and allies is probably the most important reason why the Swiss identity survived - otherwise it would probably be just a blip in European history. Swiss pikemen and their efficiency got their reputation in these times, leading to them being sought after as mercenaries by many continental monarchies at the time. This basically answers the question why Switzerland was never absorbed into the surrounding monarchies - at least for the attempts that were made up until Napoleonic times, simply no aggressor succeeded in doing it. And when Napoleon marched into the Eidgenossenschaft, its identity was long established, leading to the succeeding Swiss Republic to have basically the same borders as the old confederacy after the Napoleonic nation crumbled.

[1] "Die Schweiz als Sonderfall", Paul Widmer, http://www.nzz-libro.ch/die-schweiz-als-sonderfall.html

[2] https://en.wikipedia.org/wiki/Monarchies_in_Europe

[3] http://en.wikipedia.org/wiki/Landsgemeinde

[4] http://de.wikipedia.org/wiki/Untertanengebiet#mediaviewer/File:Struktur_Eidgenossenschaft.png

[5] https://en.wikipedia.org/wiki/Old_Swiss_Confederacy#Federation_growth

One important point is that in Switzerland most of the strong uniting factors such as language, religion and wealth are not dividing the countries along the same lines. This leads to the difficulty for secessionist movements to gather momentum. In each of the language sections you have areas richer than the country average as well as poorer. Same goes for protestants and catholics. Geography comes into play as well where all three major language areas have mountainous as well as flatter parts.

In short, the country is split by many social and even geographical divides. However, the divides are not congruent to each other and hence depending on the political issue at hand different alliances will form, e.g. town vs. countryside, catholics vs. protestants, rich vs. poor, latin-speaking vs. germanic etc.

Source: some classes at college. Prof. Wolf Linder in Berne was a political scientist specializing in development. He pointed out that when the "fault lines" are congruent on several issues, separatist movements will be much stronger. E.g. Kosovo and Serbia, Belgium but also many post-colonial countries outside of Europe.

I'm not a historian, but a swiss inhabitant, so I'll attempt to answer to your question anyway.

Seeing that you point out that we have no common language and ethnicity, I'll describe why that isn't much of a problem. The ethnicity is very much uniform, most people are christian. In the cities, there are more immigrants, and therefore other religions are mostly prevalent in those areas. There is in fact a pretty clear difference between catholic and protestant regions, as indicated by this map.

You can see that other religions cluster around the cities. In rural areas, there are very few foreigners and immigrants, and pretty much everyone is christian. You also see this at pretty much every referendum; the rural opinion is generally on the right side. This also correlates with partiotism. You can often see swiss flags and houses built after the traditional chalet style on the countryside. There is a very strong "national feeling" in these areas.

In cities, however, I have the impression that it is becoming more and more open, liberal, green, less patriotic etc. In my generation (I'm 18 at the moment), I know only one person that doesn't fit in the above criteria. However, in my parent's generation, there are much more conservatives, patriots etc.

So I have the feeling that on the cultural side of things, the "cohesive national identity" is fading away. More and more immigrants, more young, liberal, open-minded people are letting the stereotypical swiss inhabitant disappear.

I would agree with /u/tao2go about the fact that linguistic and ethnic differences are not represented in the Swiss political units playing an important role in the country's long-term cohesion. Hence, apart from a short quibble during WW1, language has never been a base for political conflict, which sets Switzerland apart from Belgium, and certainly from former Yugoslavia, where political units are/were set up on the base of language and ethnicity.

This theory is also supported by the fact that the one divisive issue that is actually reflected in the Swiss political units is Religion (in the form of Catholic vs Protestant Cantons), and guess what? The Swiss have spent Centuries of their history killing each other over this issue, all the way up to the Swiss Civil War that resulted in the modern political system in 1848.