Some more details on the unit specialization mode...
If you ever played Age of Empires (or AOEII) you surely have seen that while all faction shared most units each one was limited in building something (IE a cav faction like the huns couldn't build advanced inf and so on).
This model has been picked up as example and translated to MTW.

Each faction will get one or more of those attributes:

Heavy Inf (Danish, HRE, Almohads)
Heavy Cav (HRE, French, Spanish, Byz)
Militia (Italians, Swiss, Papacy) (this means they gets UM, MS and Halbs)
Anticav (English, Turkish)
Medium/light cav (Polish, Hungarians, Eggys)
Steppe/mounted missiles (Novgorod/Hungarians)
Advanced (HRE, Italians, Spanish) (means that they will get one or more gothic level units, lancers are counted as gothic level)
Specialist (Danish)

The classification is not yet 100% complete, but to give you an idea of how things works the only playable faction with vanilla halbs will be the Italians, making the swiss or mercs halbs quite valuable.
This is compensated by the fact the Italians gets no other heavy inf until late (and they will be GS or pikes, so no heavy duty guys), plus they can produce only light cav (again, until late)...

For a less exotic example let's take the Hungarians or the Poles, both light cav focused factions.
You'll find the spears (Poles uses the standard spear line to CS, while Huns gets AS as poor man heavy spear), light cav (mounted sergeants), FMAA and FK as usual in early.

Where the differences comes into play?
As neither one is an heavy inf or heavy cav faction you won't get the chivalric units in high, but you'll have to keep using the feudal line.

This doesn't mean you aren't underpowered, just that you can't stand in a toe to toe fight against heavier enemies and you have to change your tactics a bit (hint: your specialistic cavs got their use when dismounted too, learn to use them and you'll have some good fun especially against the Horde) or bring some extra numbers if you want to feel safer (older units are cheaper).

Without going into full details for every faction be assured they all got the tools to do combined arms tactics (Specialist factions are less suited for that but their bonuses compensate for it... I don't think you'll miss good cav very much with the Danes).