ToME can be REALLY harsh RIGHT at the beginning, sometimes getting you killed before you've explored any more of the map than the area you can see when you start the game. But that's due to randomization and generally doesn't happen much (started probably 3 dozen games, had it happen twice I think).
The first couple maps can be a bit tough, especially if you're getting used to a class you've never played before (or one of the more complex classes that you can't play till you unlock). Out of the 3 dozen starts or so, I'd say probably 50% died before completing the two intro dungeons.
However, not only does the difficulty curve even out a bit once you have some half-way decent starting gear (found in the first two dungeons), but if you do die not to far into one of the first two dungeons, it isn't like you lost that much, either in terms of effort or time (once you get the game's mechanics down, clearing both is maybe an hour, tops).
After that, the game evens out a bit till you get to about the midway, then the difficulty starts ramping up again (slowly at first, and building speed). I haven't beaten the game yet, but I have gotten very close.
And finally, if that's still a little too hardcore, there are the Adventure and (for people who donate any amount of money at all) Exploration mode where you have some (or infinite in Exploration) lives, per character. So if you die, you can pop right back into the action with the same character, items, etc, right where you died, or (usually) out on the world map (in case you got in over your head).
It also has an difficulty setting independent of the "extra lives?" setting, however, I do not recommend playing on easy, as you cannot earn achievements on easy and some of the unlockable classes and races are only unlocked by getting certain achievements (making several classes and races not unlockable if you're playing on easy).
All in all, the more "basic" classes (berserker, bulwark, archer) are pretty easy to get into, get comfortable with, and do well with. I'd recommend playing one of those three, playing on Normal, Adventure (or Normal, Exploration if you've donated and want to feel extra safe), and giving it a couple of attempts before you give up. Yes, you might die right away (or almost right away) the first couple of times, but just remember to take the game slow, think strategically, and rest whenever you're health/resource (mana, stamina, what-have-you) is below max, and you'll do fine.
EDIT: There are also some excellent resources on the forums (under the spoiler sub-forum and then split into further sub-forms by type of class) for each class, talking about their different abilities (pros and cons of both the abilities themselves and to spreading your points out or stacking them in specific abilities), what races play well with said class, and so on. If you're feeling confused about the myriad of options for each class, take a look there.
EDIT 2: A word of caution. ToME is a fantastic game, mechanically. It is REALLY fun to play with TONS of replayability because all the classes play so dynamically, fundamentally differently than each other (with a few, small exceptions). HOWEVER, the game is TERRIBLY unbalanced (which is saying a lot, cause Roguelikes in general suffer from imbalance issues due to the randomization). It is highly likely that you'll be cruising along, smashing everything in your way without a sideways glance and then some boss will come along and murder you over and over and over and over, all by himself, simply because of the way boss monsters are generated (basically they can have any abilities mashed together, so when you get into end game boss monsters, they can have abilities that would make any player basically godlike by getting the right generation of skills from a variety of different classes).
I am just about to wrap up a game of ToME. I played on Exploration mode and so I have infinite lives. I will probably have a death count of around 20-30ish when the game is over, but all but like 2 of those deaths were from dying repeatedly to the same boss, over and over and over (3 bosses total, with all the deaths split between them). I even cruised thru one of the totally optional, there-is-no-way-in-hell-you'll-ever-be-able-to-beat-this boss rooms, and still, a randomly generated mini-boss wipes the floor with me a half dozen times, all by himself.
So anyway, the point of this isn't to drive people away, it's a warning. Play the game, enjoy the game, but realize that winning is really only going to happen if you either cheat and enable god-mode, play on Exploration and just muscle thru the 1-5 encounters in the entire game that really give you any problems, or have silly silly dumb good luck. lol