I've never played Halo, but I've seen clips, is there any comparison?
I really like the "Get the Titan" map as that has a definite complexity to it. There is also the futuristic BF2 gameplay type map where you are cap'ping bases as such.
The vehicles look and sound gorgeous... the APC looks like the one in Aliens... the visual effects are amazing, such as the impact of EMP's, when flying and you manage to stay up long enough you know that you are seeing the world through a video system - and you get a feel for it being realistic. It really brings the kid out in me.
The demo is certainly a very constrained version of the full game, in fact I'm not sure if the demo will actually do the full game any justice.
I was a little unsure of the game at the outset, especially when I couldn't work out how to get off the Titan, fly the flying things, or know which way to run due to a lack of familiarity with the map... but that's what makes a game fun... when you need to learn how to play it - it makes the eventual outcome of acheiving some status all the sweeter.
And yep... there is fishing...
I've never played BF:1942, loved BF:V, am head over heels for BF:2, and I hope that BF:2142 will prove to be as rewarding as all of them put together... but it's important to remember that they are all different games in their own way.
As with BF2, it's all cutting edge stuff, and an immense amount of thought has been put into it.

Just be a nutter... life becomes much more exciting, and people won't expect anything more of you...