Is this more similar to Fantasy General or King's Bounty? or neither?
I suppose it's closer to Fantasy General. Still, it's different enough to be considered, well, a different game. In fact, Wesnoth is often mentioned as one of the few cases of completely original open source development. It's quite tactical, in any case: there's terrain - different units have different defenses on different terrain, then you've got damage types - different units have different resistances to different attacks, and the occasional special ability. Combine with around 5-15 troops (typical for the campaign, and even more on the larger scenarios) per side, branching development paths and subtle unit attributes (ie. "Intelligent" - levels faster; "Resilient" - more HP, "Strong" - more damage with melee etc.), and that's quite a mix.
Wesnoth is also a shining example of an open source project which gives graphics and even music the attention they deserve (which is often a problem otherwise, as programmers are forced to double up as artists). As Magerette pointed out, graphics have improved a lot, and in fact each minor release tends to carry something new graphics-wise. And, if you get into the game, it's quite "meaty" - the various official campaigns will surely take you a while, and even then there's still quite a bit of (quality) usermade content to go for, and, of course, multiplayer.
Ah, I ramble. In short, try it out if the "turn-based strategy" monicker doesn't immediately turn you off.