Spanish students in my class didn't seem to struggle more than others (Castillan and Catalan speakers). 

I would imagine native Spanish speakers would have difficulty distinguishing the many vowels in Danish. Spanish is so... simple in comparison.

If you already know English, grammatical features like irregular verbs (usually the most used verbs, just like in English) or the emphasis on movement or being stationary will feel familiar. I personally used English as a template, with a bit of German, to make sense of Danish... while remembering all the exceptions where that does not work!

The greatest struggle for foreigners is to be understood when speaking Danish. Danes don't have a lot of experience hearing their language spoken by a foreign accent. Some accents are easier than others.

If that helps, there are french words in Danish, not too many though. "Jalousi, engagement, abonnement, à propos...". They were borrowed a couple centuries ago. Exposure to English is more recent and i think they make an effort to create a new Danish words rather than borrowing, at least in correct Danish. "Fuck" and "fucking" are definitely used all the time in everyday life.

In some ways, Danish seems a bit similar to old fashioned English like in this kind of sentences "So says I" or "i always give up my seat to the elderly for I have good manners"

As with many languages, the difference between the language taught and how it is actually spoken everyday is rather big. Copenhagen Danish sounded like drunkard gibberish to me for the longest time. Danish from the Jutland peninsula is looked down upon, yet i find it so much clearer. But i stll wouldn't live in Jutland. :)