I hear a lot of talk about how Fallout: Tactics and Brotherhood of Steel are not canon games. I know what canon means, but what was wrong about these games in comparison to canon ones?

Tactics is semi-canon, in that the overall storyline is agreed to have happened, but some specifics aren't. There are a few contradictions with canon, and the style of a lot of things didn't match.

As for BOS, its so non-canon its not funny; but considering apparently they didn't even try its not suprising; far too many contradictions and almost proud of their ignorance of the source material.... Duck and cover explain it better than I can:

http://www.duckandcover.cx/content.php?id=11

For tactics:
http://fallout.wikia.com/wiki/Fallout_Tactics#Inconsistencies

Yeah, pretty much what's been said. With Fallout Tactics, I think it was less a result of any inherent lack of value or quality in the game, but just they simply decided to go in other directions than what was set out in that game. It's one of those things where if you played that game, you can still consider it to have "happened," insofar as it doesn't directly contradict anything that comes out later. More than being "non-cannon," it's more like "semi-canon." Like some of the Star Wars fiction, or a lot of the Marvel one-offs.

As far as Fallout: Brotherhood of Steel is concerned; beyond any lack of quality in the game itself (I actually pre-ordered it when it came out, and returned it a couple of days later, because even for the type of game it was, I didn't find it to be particularly well-crafted or - more importantly - fun to play...) the problem was really that they just didn't seem to "get" what the original Fallout games were about, in terms of overall tone and setting.

If I were watching someone play Fallout 3, and didn't know anything about it, I'd still fairly quickly be able to say "Hey, that looks kind of like a Fallout game." Brotherhood of Steel came off (to me, at least) as just another generic post-apocalyptic pastiche, devoid of many of the specific characteristics and underlying themes that were so crucial to the Fallout franchise at the time. They took a lot of creative license with BoS; most of which wasn't for the better...

A certain amount of creative licence is allowed in a follow-up game, and yet it can still be canon to previous games. Canon being a general rule or principle by which something is judged. Players can sometimes get bogged down with fine detail that is not identical to the previous, and think that it is not canon.

Fallout 1, 2 and 3 are indeed canon to each other, with the distinctive game-play feel of being Fallout, with it's mature content interspersed with shocks that are sometimes of a distasteful nature, along with similar Fallout type of character interactions and overall strategic game-play. The balance of content of all three Fallouts are similar.

This is where Fallout Tactics differs, it differs by focussing largely on the strategic and tactical play that is of a more intense nature, and with a need for a more calculated combat play.

Fallout Tactics was a fantastic game, as is Fallout 3, but Tactics had not the same balance of content of the other Fallouts to be judged completely canon.

I don't really think that different gameplay (Fallout 1 and 2 were RPGs, and Tactics was a squad-based strategy game,) is something that's going to disqualify a spin-off title as "canon." I don't really see as how Tactics, by focusing on... well, "tactics" had any effect on whether or not it was later going to be ret-conned as "semi-canon."

With the word "canon," in the first place - we're inherently talking about storyline elements. Gameplay elements don't really have anything to do with whether or not a crack squad of Brotherhood of Steel fought against Supermutants and a sentient supercomputer during the events depicted in Fallout: Tactics.

Some said, when Fallout Tactics came out, that it was really Fallout 3, (at that time). Though with less of the mature storyline. Call Fallout Tactics "canon" to Fallout 1 and 2 if you like, and as you suggest, along with the current Fallout 3 storyline of game-play and elements. All of them being RPGs.

Though it feels, when playing Fallout 3, that it is more akin and canon to Fallout 1 and 2 than Fallout Tactics ever did.