I always feel worried to post such a topic as I feel I'd be seen as non-committed and not-serious about HRT, etc, etc, especially if someone I knew who deals with me at the NHS read it.
MugwortPsychonaut - I'd be scared too, and in fact if/when I do go on HRT I'm scared witless as to what will happen! There's just no telling what will happen psychically and mentality. Like I said to teh NHS therapist "I know I'd be essentially playing Russian roulette with my body, and I could end up in a worse state than when I started." And he naturally agreed, as that's exactly what you're doing. Personally myself I think it should be easier for doctors to predict what will happen to you, but of course, such testing for each transexual patient would cost more money, so I presume on that basis - doctors would rather they take your chances and just see what happens. So, there's alot to be scared out!
I have no idea what tingly nipples would feel like, or any of the other physical feelings most likely. But I can easily empathise with your concerns regarding diving into the whole experience. Hang-on in there whilst your more sure than un-sure that you're doing the right, or most probably right thing.

Hope it all goes well for you!
My biggest worry is becoming sort of "stuck midway between the floors" of male and female, physically and mentality speaking. A loss of all male function, metabolism, turning overweight and becoming over emotional, yet not feminizing well facially or with regard to healthy female body-fat distribution, would be the absolute worst case scenario; A gender and biological worst-of-both worlds. There needs to be more pros to outweigh the cons, otherwise it'd be madness for me to continue down this path.
If I (and we for that matter) are to trade the (relative) perks of being physically male (such as faster metabolism) and socially male (some receive male privilege), then there does have to be some perks to taking HRT. Otherwise if you're likely to end up in a worse physical and mental condition, and yet you still continue then that is perhaps bordering on a form of insanity. Its pointless changing if your life won't be any better, is my logic. That's what I keep telling myself anyway.
Taking the experience day-by-day and evaluating whether this is right at the end of each day, is sound advice.