I hear that Microsoft is offering Windows 10 as a free upgrade to eligible users. Will this be a full usable operating system or will there be a catch somewhere down the line?

It will probably be full, but there are a few things.

First, I believe they are moving to the "app store" type of sales. As in, you will be able to get in the windows store and buy any software you need there.

Second.. I heard that most software (as in, photoshop etc.) will go subscription mode. Which will probably need a valid windows 10 copy.

And third - Microsoft has been trying to get their OS on mobile for a while, without much success. Windows 10 might just be able to do that, they have learned from their windows 8 mistakes, I trust. This might mean free(or paid) cloud, shared between all your windows 10 devices, which would be great.

Just off the top of my head. :)

I received the offer from Microsoft and registered for it (a requirement). It will automatically download when it's released and it's going to be the full version, not a trial

I saw this coming back In the 90's, at work we had a LAN with MS office on the server, (and Word Perfect) . I had a 5Meg hard drive, that I could never fill. Only things on the hard drive was windows, access to the server, and my files. 

With tablets and computers you will be using programs online (and paying for the use) and possibly storing your files online. No need for a hard drive at all. Just a basic windows stored on an EPROM on each computer and a mouse and printer drivers. (possibly you will not need printer drivers ether)