If Manchester Utd, City or Chelsea where currently sitting in the position that Arsenal find themselves in, 4 points ahead of second place, then people would be saying that they are in with the best shout of winning the title and that only their own mistakes would be the reason for them missing out on it. However, with Arsenal top of the league, people are still saying they are not favourites to win the Premier League this year, it is almost assumed that they are going to mess up somehow at some point. Manchester City are even still odds on to win the league despite being 4 points behind the Gunners.

Where Arsenal have failed in the past is when it comes to beating teams they really should be beating, they often drop points. At the very start of the year they lost 3-1 to Villa and people believed the tradition would continue, however after their 3-0 win away to Cardiff, a different tune is being sung.
Let's not forget, Cardiff beat Manchester City and achieved a draw with Utd. Southampton beat Utd and Liverpool but both of these teams suffered in the hands of Arsenal. And the win over Cardiff was nothing short of a hammering, and what's more, in style.
This was Arsenals third clean sheet in four league games, showing their defensive problems appear to be over. Their midfield is incredible, I would say the best in the Premier league especially with Aaron Ramsey being in the form he is in currently. Arsenal have now also scored 27 goals this season, they appear to be the real deal.
A lot of the recent success can be put down to Mesut Ozil. This signing has done so much for Arsenal, firstly because he is a wonderful player and has been assisting for fun since signing the dotted line, but also for the change of mood he brought to the club. He excited fans, the other players and has made everyone up their game. Fans had long been crying out for a big name signing after selling Fabregas, Nasri, Van Persie etc. and now they have one, Arsenal appear to have turned a corner.
It is true that Arsenal have not really had to face the toughest of opposition, although they did pick up a win over Liverpool, they lost to Utd and still have to play the likes of Tottenham away, Chelsea and Manchester City in which their season could be turned around completely. However, they have shown they can do the business against the big boys having dominated their group in the Champions League, containing last years runners up.
Arsenal have 11 different scorers this year, joint most with Everton, which shows they have a good team. Often it is the teams who share the goals around who do well, unlike teams such as Liverpool (Suarez) and Manchester Utd (Rooney) who look to one man to score all their goals. However it is undeniable that Arsenal need to sign a world class striker.
Giroud has put in some good displays this season, but he is not as prolific as you would expect of a title winning striker. The only back up Arsenal have is Walcott, Podolski and Bendtner, none of these have set the Premier league alight. If Arsenal were to sign a Rooney or a Suarez they would have a terrific chance of wining the Premier League, this would make a statement that they are in this to win and they are going to achieve that goal.
While there is still a long way to go, I think Arsenal have shown enough for people to take them seriously as title contenders. I may eat my words come May when they are celebrating a fourth placed finish, however I feel Arsenal can for the first time in years, realistically win the title.