Ok, so it's been about seven months since I started learning Welsh in earnest, and he's what I think is hard and what I think is fairly easy about Welsh:
1. Mutations. Letters change at the start of one word depending on what word they follow. However I'm finding that this starts to feel more natural in most circumstances after a while.
2. Versions of "about" and "of/from" for each pronoun. If you're talking about a thing, they're "am" and "o" respectively. But if you're talking about/from me, you, her, him, us or them you're looking at "amdana, amdanat, amdani, amdano, amdanon and amdanyn" and "ohono, ohonot, ohoni, ohono, ohonon and ohonyn"!
3. The letter "Ll". Yes, it counts as one letter. The pronunciation is somewhere between ell and hissing like an angry goose with a headache. For some reason I find it easier to pronounce at the start of a word, like Llawr, than in the middle or end of a word, like Arall or Galla.
4. Conjugations. Well, this is the same for any language, really. Remembering the conjugations of short-form verbs is just one of those things you have to get used to.
5. Yes and no. There are no words for Yes and No in Welsh, you just repeat back (or negate) the appropriate conjugation of the verb in the question. "Do you like cheese?", "I do". "Are you tired?" "I am". "Are you reading?" "I am not". But that's only in the present tense. In the past tense it's usually "Do" for yes and "Naddo" for no. But then for emphatic questions it's "Ie" for yes and "Nage" for no. If the question was asking if there is something with you (see below) you reply with Oes or Nag Oes. Remembering which to use is the hard part.
6. There aren't many words that are similar to other European languages. There are some, mostly more modern words borrowed from English: tacsi, parcio, ffatri, ffôn, beic, etc; and some ecclesiastical words like eglwys (church) and capel (chapel) but not to the extent that you find with Romance or Germanic languages.
Things that are easier than I expected:
1. Mutations. Yes, they're difficult, but actually you get used to them, and in many cases the rules are quite consistent. You just get used to dropping the G in words like gweithio and gweld when they follow a sentence subject. You get used to changing D to Dd in adjectives after "yn", and you get very used to nasal mutations after "fy".
2. Conjugations. Although remembering them is difficult, many of the conjugations are quite consistent and actually fit in quite well with the pronoun they go with.
3. Possession. I've said before about there being no form of "I have a (something)" in Welsh, they instead say something is "with them": Mae ci gyda fi - "There is a dog with me". I got the hang of this much quicker than I thought. I think you just change your way of thinking and everything slots together.
4. The language as a whole. I'm only a beginner. Very much so. But I've been surprised at how much I've covered in such a short space of time. I'm not proficient by any stretch of the imagination, and I can only string together fairly simple sentences, but on the other hand I can make many more types of sentences in Welsh than my partner's daughter can in Spanish; a language she's been learning at school for two years now. I can use the present tense, the past perfect, a sort of imperfect, the future in several forms and some conditional tenses and even a kind of limited subjunctive mood.
On the whole, Welsh has (so far) been easier than I expected. It's not the incomprehensible jumble of letters it always looked like on road signs when we used to visit my Grandma in Rhossili when I was a child. In fact, in many ways it's a lot easier than the languages I'd been taught at school.