Usually, to figure out the things that I like in a TF you can ask these questions:

1. Does the subject gain body parts I don't have (paws, snout, tail, etc.)?
With this point, usually the more things I personally don't have, the more I'll like it, as this is mostly due to my TF fetish coming from having an intense curiosity of how other parts would feel like.

So in that regard, things like Human to Human TF or Human to Kemonomimi (Ears + Tail) don't really do anything for me. There are some rare exceptions to this of course, but it's usually because they have more going on in other places.

Maractus for example doesn't really have a snout or anything, but it doesn't have legs, hops everywhere to move, and its natural expression is a smile. I'd like to imagine it requires more effort than normal to frown with that mouth. Also, it's cute. Cuteness is important.

Of course with many things I have a limit, so not everything I don't have works for me. There are some situations I like, but I'd rather not know what it feels like personally.

2. Is enough changing?
This point is mostly about physical changes. I have a much higher preference for Human to Furry TF rather than Furry to Furry TF for example.

A human growing/grew a snout, fur, and radically different ears? Great.

A creature that already has a snout getting a slightly different snout? My desire is practically non-existent (in most cases).

I feel a little better with the previous situation if their body material is changing.
(To a Plush/Inflatable)

I typically draw post TF so the starting form could be left to interpretation, but I usually prefer to imagine the starting form is a human male. Of course, an Ex-human who then turns into a different furry is 100% fine to me, because at some point they were human so it's just more changes from the baseline.

Also other things I like coupled with the previous statement are situations where the person becomes shorter or a loss of dexterity due to less/unwieldy digits so mundane tasks are harder to do.

3. Would the TF cause esoteric and hard to describe sensations?
This one is pretty big. Things like a living plushie or a suit having a desire to put things inside its zipper, or an inflatable having air passing through its air valve. Does it feel good? Is it overwhelming to the point of addiction? All good thoughts.

I explored this concept a little in one of my comics titled "Marketable Maractus Merchandise".

4. Do I find it ""cute""?
Honestly, you could probably call this one more of an "artstyle" thing.
There are TFs I like that aren't cute appearance-wise (I drew CTF once for example, and I really wouldn't call that cute) and situations I would actually just call plain old horrific if you take a moment to think about it, but for the most part I have a high preference for cute things. I have a strong distaste for anything "realistic" looking and lean more on the cartoony side of things.

As for the other bit of the question...
I'm not terribly into merging. Maractus is cute, but I'm not into every situation involving Maractus.